I'm a bit confused on this buying system. Does buying ammo add on to the cost of your gun, or can you just buy ammo?
No, it doesn't add to the cost of the gun. As long as the purchase DC is lower than your wealth bonus, (and with a +18, it probably is) you don't even have to do anything. You can afford it. There are a few things that cause your wealth bonus to decrease, and those are in the wealth section of the MSRD.