Looks like quite a few of us were typing replies on this thread at the same time. Regarding sheer poles - here's what James Lees's The Masting and Rigging of English Ships of War, 1625-1860, has to say about the subject (p. 42):
"To prevent the shrouds from twisting it was the practice, especially during the nineteenth century, to lash an iron bar across the shrouds just above the upper deadeye; this was known as the sheer pole. It was carried on the outboard side of the shrouds and was about 2 inches in diameter."
That's pretty vague - and of course the author is only talking about British warships. When I was working on my little model of the Hancock I tried to figure out whether an American frigate of 1776 would be likely to have sheer poles. After looking at all the contemporary pictures and models I could find, my answer was "well, maybe." The sheer pole seems to have come into use gradually between the very late seventeenth and early nineteenth centuries.
That the Victory currently lacks sheer poles just establishes that the eminently-qualified people currently in charge of her don't think she had them in 1805. I think they're probably right. George Campbell apparently agreed; his drawings, reproduced in the Longridge book, don't show them. Neither does J.M.W. Turner's famous Trafalgar painting, which is known to have been based on sketches he made on board the ship shortly after the battle. (That painting and the surviving sketches open up several interesting cans of worms. They suggest, for instance, that Heller was absolutely right to omit the ornamented entry ports from the ship's sides. But that's another topic.) If I were building a model of the Victory I wouldn't give it sheer poles. But I also wouldn't argue violently with anybody who did.
I'm pretty sure the famous Isaac Hull model of the Constitution does have sheer poles on the lower shrouds. (I say "pretty sure" because I've misinterpreted photos of that model more than once.) That model apparently dates from 1812 or shortly thereafter, and is generally regarded as the best source of information regarding the ship's rigging during that period.
Iron "ratlines" were quite common - though not by any means in universal use - in American and European merchant ships beginning in about the last quarter of the ninenteenth century. (I don't think there's any firm date for their introduction, and, as I said in my earlier post, plenty of ships never used them.) Some years ago I was boning up on the famous British merchantman Torrens, with the intention of building a model of her. (That model, like most of my big ideas, barely got started.) I found an excellent photo of her showing quite clearly that (at least at the time the picture was taken) she had iron "ratlines" on her lower masts - but that the rope ones had been left in place. (She looked kind of weird. The rope ratlines only stretched across the two center shrouds of each gang, and the iron bars, stretching from the first shroud to the last, had been lashed on top of them.) The Cutty Sark, launched in 1869, never, so far as I can tell, had iron ratlines. But many big and medium-sized merchantment were using them by that time, and their popularity increased as the years went by. They were, as GeorgeW suggested, extremely common in American whalers. Some of the big steel square riggers of the early twentieth century had rope ratlines; some had iron ones. (Iron "ratlines," so far as I can tell, never became popular in warhips.)
It's easy to see why there would be some disagreement on the subject. Iron ratlines would be durable, and wouldn't require much maintenance. On the other hand, a rope ratline, sagging a little under the weight of the person climbing it, might feel a little more secure - and would be slightly less dangerous if wet or covered with ice. And iron rods cost considerably more than rope.
If I were building a model of a latter-day sailing ship, I'd search long and hard for photos of the prototype before deciding whether to make the ratlines of iron or rope. Iron ones, by the way, are remarkably easy to represent on small-scale models. A nice, straight piece of blackened wire, adhered to the shrouds with tiny spots of adhesive, will do the job quite nicely.
The Model Shipways Kate Cory kit, unfortunately, has been out of production for many years. (The company has been making rumbling noises about bringing back many of its old solid-hull kits; I hope that will be one of them.) MS does currently offer a plank-on-bulkhead Charles W. Morgan. Those Model Shipways p-o-b kits occupy a completely different planet than the HECEPOB kits; the Model Shipways designers understand scale modeling. A couple of guys in our model club have built that MS Charles W. Morgan, with extremely impressive results. Of the currently-available whaler kits, that one gets my highest recommendation. Be aware, though, that it's a big, expensive, complex kit - definitely not intended for newcomers.