Now THIS is up my alley! :)
Ok, first up, my favorite configuration: 5 Auto-Rifles and 2 Intek Laser Rifles per squad with cheap Divisor 2 armor (either the Lyran or Davion pre-clan vests will do). It gets them 3 points of damage at 3/6/9 range with no movement penalty (since the Inteks aren't support weapons). Solar rechargers are very cheap, and Inteks are the most efficient laser rifles around, even before you customize them (they start at 2 points per shot, and can be dropped to 1 per shot for triple the cost).
Second, a higher damage configuration: 6 Auto-Rifles and 1 Auto-Grenade Launcher (or Heavy if you can get them), again with the Divisor 2 armor from above. It's 5 points of damage per squad at 1/2/3 range and no movement penalty.
Third, the REALLY high damage configuration: 7 Auto-Rifles per squad, again with the cheap Divisor 2 armor, but now LAW Rockets for everyone! Of course, it's only one shot, but it's a very bad day for any 'mech on the receiving end of it, and it's still 4 points of damage per squad even after they've fired the LAWs. AND it's all without a movement penalty!
The really long range stuff (e.g., Heavy Support Lasers) is probably out of your price range, so I'd stick with the above...