The first step before buying any physical goods is to check if there are any groups or LGSs that play BattleTech within driving distance. If not, you might as well swing full digital and either play over Megamek or Tabletop Simulator.
Anyway, since you already have the Beginner Box the next step would be
A Game of Armored Combat if you want the minis and the other physical goodies. If all you want are rules,
Total Warfare will give you rules for most things you're likely to see on the board, and
TechManual will give you rules to design
your own BattleMechs and vehicles (as well as a lot of neat background info on the technology itself). Both of those together should be about 20 bucks cheaper than AGAC, but you don't get anything else with them, just the books.
All that said, you could get all three things I linked for less than it would cost to build a 500-point Warhammer 40k army.