[tolbiny]

The best combination of 2 upgrades is weapon and stealth. Stealth lets you spy and find out who has what level of fortification, but since you can't attack much without a weapon its worthless on its own.
Buy stealth first and attack any bases with no fortification upgrades untill your wanted level gets to 4-5. then you start losing to much money on your attacks, and you should just search mountains untill its reset, then repeat. Once you have enough for rock (good ole rock, nothing beats that) you can attack any base with 0 fortification without any care of your wanted level. You will have enough from one full day of attacking with weapon 1 and stealth 1 to get a fortification and a security upgrade.
don't upgrade transportaion unless you plan on mapping the area and really investing the time.