If you want to get a ton of minis on the cheap buy the following
Battletech Intro Box 26 plastic quality minis (which includes two high quality, currently a Battlemaster and Timber Wolf)
Alpha Strike Lance packs. All plastic, 4 per box, but same quality as the new intro box. There are 8 total I think, listed below @ 19.99 each and come with nice quality alpha strike cards (very nice, full color).
Fire Lance - Shogun, Trebuchet, Dervish, Stalker
Command Lance - Cyclops, Cataphract, Catapult, Ravem
Battle Lance - Zeus, Banshee, Hatchetman, Orion
Support Lance - Awesome, Vindicator, Charger, Firestarter
Recon Lance - Spider, Vulcan, Cicada, Flea
Striker Lance - Dragon, Quickdraw, Guillotine, Panther
Pursuit Lance - Wolfhound, Clint, Jenner, Centurion
Assault Lance - Atlas, Victor, Grasshopper, Blackjack
As for where stuff comes from...
The unit quirks are from Strategic Operations
The pilot Special Abilities are from A time of War (RPG)
AS in its simplest form was adapted, or expanded, from the quick strike rules found in Strategic Operations.
If you don't really want to play anything from the Dark Age you can always pick up the new First Succession War book that was recently released in PDF (and print by the end of the month).
You can also use
http://www.masterunitlist.info to build and print your own forces alpha strike cards.
For other product info you can browse sarna.net to get up to speed on what books you have missed.