The ones Atlanta Cutlery and International Military Antiques (IMA) sell as "cleaned and complete" are generally pretty nice. I've bought two dating from the 1880s, a short-lever and a long-lever, and they both had decent bores and headspace, and are actual shooters. Some of the "as found" rifles they sell as clean-them-up-yourself projects may have bad stocks or missing or broken parts that will require repairs, and may or may not wind up usable. The Martin-Henrys are all British arsenal made and proofed, the Nepalese made Gehendra and Francotte copies are more problematic, and generally in worse condition, and harder to find parts for. I wouldn't trust the Francotte as anything other than a wall hanger, but some of the Gehendras look okay, but will probably require custom handloads, as their bores were done by hand in Nepal, and not by skilled machinists in England. Restoring a dirty, rusty old rifle to presentable condition is a worthwhile project in its own right; if you get a shooter out of it, that's just a bonus.