Sell all weapons you picked up and like aurlaent wrote don't buy phase shield until you need them.
What I did and recommend to every one is to buy the turretgun first, it's a real moneymaker as it lets you hit targets and you can concentrate on avoiding getting hit.
After that buy the laserturret which is even better although it can only hit airtargets and last get the last beamweapon (the most expensive item in the game).
You can also buy the plasmagun and the micromissiles as they are always equipped.
The rest of the weapons aren't worth it imo.