I guess this sort of thing depends on how concerned one is about scale accuracy. To the eyes of many ship enthusiasts the shape of the
Bounty's hull is instantly recognizable. That actually may not be quite fair; she was a thoroughly typical merchant vessel of the 1780s, and probably looked quite a bit like dozens, if not hundreds, of others.
A slightly less obvious choice for such a conversion project might be the Airfix
Endeavor, if you can find one. (It's been out of the catalog for a long time.) There again, though, we're talking about a hull shape that lots of ship enthusiasts would recognize immediately.
There is of course no such thing as a typical hull shape for a brig. Brigs came in many different shapes and sizes. The term "brig" refers to rig - not hull form or function. Some brigs were naval vessels; others were merchantmen or privateers. (The
Bounty's hull form has "merchantman" written all over it; she'd be difficult to pass off as a naval vessel.)
At the moment I can't think of a single plastic kit that represents a brig with reasonable accuracy. Heller used to produce a kit labeled "brigantine," but it was based on - believe it or not - the hull of the steam exploration ship
Pourquois Pas? Back in the Olde Dayes, Pyro made a tiny version of the brig
Fair American, but it didn't really rise above the toy level. And back in the Even Older Dayes, Aurora made a totally fictitious "pirate brig" called the
Black Falcon. I guess it might be possible to turn that one into a believable scale model of a brig, but I'm inclined to thing scratchbuilding would be quicker. Maybe I'm forgetting a kit somewhere; I hope so.
Several nice wood kits representing brigs have been released. The Model Shipways
Niagara, Volante, and
Fair American stick out in my mind immediately. (The various European kits claiming to represent the Revolutionary War brig
Lexington are to be avoided. That's another story.)
For scratchbuilders, one of the finest ship modeling books ever published deals with a naval brig:
Modeling the Brig-of-War Irene, by E.W. Petrejus. It deals in great detail with a British brig that wound up in the service of the Dutch. Unfortunately the book has been out of print for years, and the few used copies I've found on the web have been extremely expensive. Oh, for a new, English-language paperback edition of that book!
All this serves to emphasize the meager breadth of the plastic sailing ship kit market. A brig from virtually any period, with its two square-rigged masts, would be an excellent "intermediate" project for somebody getting into the hobby. The wood kit manufacturers figured that out a long time ago. The plastic kits manufacturers apparently don't care.