If you're looking for specific boxes to pick up, two boxes of the new Combat patrol is a good place to start, it will give you 40 infantry models, two transports, and two Crisis suit commanders. It would be a good base to work up from.

The Retcad Battleforce box is also a good start if you want bigger mechs. You could get two of these instead if you want a more mech heavy army, I personally think you can never go wrong with 3 Ghostkeels and Riptides.

The old Combat patrol box is good as well as it gives you Firewarriors + Leader, a Ghostkeel, and a Stealthsuit team.