i would do as cayuga says and scan the auction sites (auctionarms.com and gunbroker.com) for older t/c rifles. the new englander and the renegade will probably shoot about the same, but the stock designs are a little different. the new englander has more drop in the stock. also, the new englander has a round barrel. the white mountain carbine is a octagonal-to-round barrel. the renegade and new englander both have 26" barrels (unless you get a rynite-stocked new englander or a greyhawk). white mountain carbine has a 21" barrel. there are two types of renegades, one with double-set triggers, and one with a single trigger. (i have the double-set version, which is very nice).
new englanders all have a single triggers. my uncle's new englander, manufactured in the late 70's, has a rather poor trigger pull (but tolerable). my greyhawk (a stainless new englander), manufactured in the early to mid 90's, has an outstanding trigger pull.
.54 can shoot a larger roundball that carries more punch, if you are planning on hunting with roundballs. otherwise, a .50 provides more bullet options. i have a renegade in .54 which shoots strictly roundballs. also have the above-mentioned greyhawk (same thing as a new englander, except it has a 24" barrel, is stainless steel, and has a rynite stock) in .50. got both of these rifles for really good prices