I would figure one of the 55 Ton designs, Griffin, Shadow Hawk, Wolverine. Most would have been manufactured in the periphery at one time or another, maybe even currently, so there would be some supplies
LRM's, SRM's and AC ammo are pretty easy to come by, and even manufacture in the periphery. If you want something less ammo dependent, there are versions of the 55 toners that swap the AC for Large Lasers or PPCs.
The one thing you may not be able to do is keep your mech in one standard configuration, you might have to swap an AC for a PPC or Large Laser, or a LRM with SRM rack or vice versa, or with medium lasers or Heat Sinks, all depending on salvage and supplies.
275 Engines are fairly common, so are the 3 ton Gyro that goes with it.
However after the Clan Invasion, the days of mechwarriors being cross trained as techs I think were over, as were the days of the tech able to scrounge together enough parts make a mech and to become mechwarriors.