With the situation your in I wouldn't do it. Yes it is very tempting to get a terminator and who wouldn't want to own but you will be getting yourself into deep pockets. Modding one would be highly addictive too
It's going to cost you a lot more than that for a Hellion kit to make that safely. Remember, the motor in your 99 Cobra WILL NOT live @ 475-500 rwhp. You will be putting in a new motor at that power level. The block is the only thing good in a 99 for that power level. All the supporting mods are not up to par.
With the 04 you can usually find one with a pulley, exhaust and tune thats making in that range and live a long life without even stressing the engine.
Plus, if you dump another $8000 into the 99, it will not be worth the money of the 04. If you ever have to get out of it for some reason, it will be a lot easier to get your money out of the 04 then a modded 99. Just my $.02
AND????? It will give me a reason to rebuild it and turn the boost way up. Then I'd be 550-600rwhp. lol 1 month and running great. Modified Mustang is sending a photographer to take pics for a feature in thier mag. They turned down a friends 04 Cobra. I love 03-04 as much as everyone but I went with a 99 because it left more fun money, and I could pay cash. No payment is awesome!