I did a lot of digging into the history of this ship back in the late seventies, when I was working on my little model of the Bounty as she appeared (I think) at the time of the mutiny: http://www.hmsvictoryscalemodels.be/JohnTilleyBounty/index.html . That was a long time ago, and my memory is...well, to be charitable, let's say notoriously inconsistent. But I do remember some points that may be of help.
To begin with, I've never seen any evidence that the Bethia was a collier. She's generally described simply as a "ship-rigged merchantman." I may well have missed something (I wasn't particularly interested in the ship's pre-Navy career, and quite a few of the reference books that are now common hadn't been published when I was working on that model). I know Cook's Endeavor started life as a collier; maybe the Bethia was one too, but I've never seen a contemporary statement to that effect.
Here's a link to a post on another website, in which I summarized some useful information about the ship: http://forum.drydockmodels.com/viewtopic.php?f=10&t=1339 . That thread started in 2005, but I don't think I've encountered any information since then that contradicts it.
In terms of plans, the bottom line is that there are two sets of contemporary "Admiralty Draughts." The first apparently was drawn on the Navy Board's orders right after the Bethia was purchased. (Genuine, contemporary plans of British eighteenth-century merchant vessels are as scarce as hen's teeth. We're lucky we have this set - courtesty of the Royal Navy.) I think what happened was that the draftsman drew a more-or-less complete set of plans of the Bethia, and then (as is noted in the documentation that accompanies them) added some "proposed contrivances" in red and green ink. (Admiralty draughts quite frequently were drawn in more than one color. Unfortunately the colors have long since faded almost beyond recognition - and, of course, they don't show up in black-and-white reproductions.) I think (here's where my memory may well be playing me false) the "proposed contrivances" included the capstan, the gunports, and the swivel stocks. (My thinking is that the capstan, in the presence of a relatively large Navy crew, would be regarded as a more efficient apparatus for handling the anchors, and other heavy jobs, than the original windlass up forward - though the latter was retained.) There probably were some others; I don't remember.
Then, apparently just before the Bounty sailed for the South Seas, another set of plans got drawn. These are the nearest thing we have to an accurate impression of what she looked like when she entered the history books. (There appear to be no contemporary sketches or paintings of her. That's not surprising. When she sailed for the Pacific, in December of 1787, she would have been regarded as a thoroughly unremarkable vessel - and she never came back. The oft-reproduced engraving showing the mutineers throwing breadfruit plants at Bligh as he rows away in the launch obviously was made after the mutiny, when reference to the actual ship was impossible.)
The second Admiralty draught shows lots of interesting stuff. The little deckhouse that originally stood aft of the capstan was removed. A new enclosure was added next to the taffrail. (Mr. McKay says it's a "flag locker." I say it's a water closet for the captain, who'd been evicted from his cabin to make room for the racks holding the breadfruit pots. In any case, a box covering the tiller head was incorporated into the front of it.) A series of holes (for ventilation, muskets, or both) were bored through the hatch coamings. A little stove was installed in the great cabin to keep the plants warm. And, of course, a complex set of racks for the plant pots were installed in the great cabin.
A few more bits of information relevant to this question can be gleaned from contemporary correspondence. She did indeed carry studding sails (though Bligh concluded that the lower ones were "too long, so I cut them and made a royal out of the canvas" - which is why my little model has a royal on its mainmast). Whether the studding sails were present while she was in the merchant service I don't know; my guess is no. He asked that she be fitted with "gratten tops instead of boarded." (Presumably that means the "open tops" that were standard in the merchant service.) And she was fitted with "a pretty figure representing a woman in a riding habit." (What her figurehead was while she was in the merchant service - if she had one at all - I have no idea.) And her bottom was coppered after the Navy bought her.
A couple of warnings about the Revell kit. The shape of its hull is generally quite good. But somehow or other the kit designers really botched the profile of the head knee - which is pretty vital to the appearance of the ship. It needs to be removed and replaced with one cut from styrene sheet. (The job takes about half an hour.) The windlass is shaped wrong; the two Admiraly draughts make it clear that it was a simple, octagonal spar with no taper (but with three pawls in the pawl post, so it could be rigged to turn in either direction). And the transom probably is in error in showing a odd number of genuine, glazed windows. (Bligh's log says that, in a storm off Cape Horn, a big wave "struck the stern and stove all to pieces between the cabin windows where the sham window is." Implication: the window in the middle was a fake. Glass in that position would only serve to give outsiders a view of the rudder stock.)
Those are the big (or relatively big) points that I can remember at the moment. That thread from Dockyard Models contains some more. By the way - the presence of those two Admiralty draughts has confused lots of people over the years. My opinion of the Anthony Hopkins/Mel Gibson movie, "The Bounty," is generally pretty high (though the scriptwriter didn't do his homework), but I get a laugh every time I watch the scene near the beginning where Anthony Hopkins is sitting in his cabin with a copy of the ship's plans behind him. The print is entirely in black ink, and it's obviously a copy of the wrong set of Admiralty draughts - the one showing the Bethia before the modifications were made.
There have been three modern Bounties, all made for the movies. The one in the 1932, Charles Laughton/Clark Gable version was converted from an old schooner; it looks about right from a distance, but that's about all. For the Marlon Brando/Trevor Howard version MGM commissioned a firm in Nova Scotia to build a ship from the keel up. It was more-or-less based on the first Admiralty draught, but was deliberately made about twenty feet longer than the original to accommodate the Cinemascope cameras and Mr. Brando's ego. (It was also, for reasons I've never been able to figure out, painted bright blue.) And most of the deck furniture on the 1959 replica is irrelevant to reality. The one built for the Anthony Hopkins/Mel Gibson version probably is the most accurate of the three - but it doesn't have the little water closet or various other features that are shown in the second Admiralty draught. (It does, however, have running rigging made of some almost pure white material that, to my eye, really damages the illusion.) Bounty #2 has shown up in several more recent movies, including the Charlton Heston version of "Treasure Island" and at least one of the "Pirates of the Caribbean" flicks. Bounty #3 appeared in the Turner Cable Network's mini-series on Captain Cook (would that it would come out on DVD), sometimes, through the magic of special effects, representing the Discovery and Resolution simultaneously. I don't know what's become of it recently; the last I heard it was said to be on the verge of falling apart, due to shabby materials and construction. But that may have been just a rumor.
Hope that helps a little. Good luck.