UBCS, check the MOD info tab on the side there. Under factions you'll see the 'Mechs available.  The rule for choosing them is money, everything will have a price.

Don't you mean C-Bills, not Z-Bills?

Oh, damn... Kit, I'm sorry, man... I didn't mean to spring your trap.  Man, you gotta warn a brotha, here, hehe.

Man, if Bosnia taught me anything, there's no such thing as a wasted mine.  Someone will always step on it, eventually.

No, the mechs in MWLL have been chosen from numerous periods in the progression of Classic Battletech. Some of them, like the Flea, Warhammer, Marauder and Atlas are old designs from the 1980s. Others, like the Uziel, Thanatos and Mad Cat MK2 were introduced fairly recently in Mechwarrior 4 about 8 years ago, and there's different chassis from all the way in between.

Some were chosen because they're iconic, others are there to fill out the table. All of them are fairly well-known though. The vehicles not so much; they were chosen specifically to fit a certain battlefield role.