It depends on exactly where you are in terms of enemies. However, if you can, the Heavy Tritanium Suit costs a bit more alloy, but essentially makes your agent like Robocop against conventional and laser weapons. Nothing gets past it -- not gunfire, not explosives, not the dreaded gas grenade.
I would also save just enough for a single tank (Laser is best, but if you don't have it, rocket works well too). It is super good against Ghost missions, which will be popping up.