Man if I was only paying $200-300 every 2-3 months or so I'd be in heaven. It's been more like 200-300 every two weeks or so!
Wow! That is a bit steep. OK, a couple questions:
1) Do you have the tools/know how to work on the car?
2) How long still for school?
I totally understand what you are saying about the costs and such, but despite my car being in very similar condition when I got it, I've put very little into it relatively speaking. To be succinct, I've put $3k on top of the $3k I spent for it. $1500 is from the rear end and drive train components that I replaced (I don't think I actually needed the half shafts, drive shaft, or bearings, and the diff was an upgrade). The ~$700 on the AC was just b/c I'm a pansy and can't do without it. The rest is pure luxury upgrades as I want my car to have all the toys of the 325is and then some (I've had 3 E30 325is')
So the reason I asked question #1 is that if you have the tools and can make use of this forum and other E30 forums, there really is nothing you can't do. Also, you are in SoCal where there is a HUGE E30 support system who can provide anything you are lacking.
#2 is to determine how long you can hold out financially for this car. As previously stated, you will have the new car payment for much longer and will be grossly upside down for a long time, thus in the long run costing a significant amount more than the 318. So if you can afford to resolve some simple drivability/reliability issues, then just hold on to it and when you graduate, make it the way you want. I'm currently finishing my FINAL class for my masters (thus why I'm procrastinating on a couple papers and writing long responses on stuff like this) and once I'm done will begin to finish getting mine to be just what I want.
So to dig a bit deeper, what are the remaining issues with the car? It sounded like you know what to do to fix the idle/AC issue and that is really all I saw as being critical for drivability. The PS leak is important, but that is a couple hundred new and a good bit less used, and really is not critical. I drove my first 325is until I sold it with a PS leak. The rest of the stuff is cosmetic (seats, dash) and can wait until you finish school and want to complete a restoration.
Over the years, the ONLY reason I've been able to keep so many toys is b/c I've learned how to do all the work myself. The only new vehicles I've kept for myself were new motorcycles (I currently have a pretty healthy payment for a beemer).
So that said, give us the poop on what needs to be done and lets get some hard numbers out there so you can make a better informed decision. We looked at a Fit for my wife a while back based on the mileage she puts on cars and just couldn't bring ourselves to get one of the cheaper models due to it being a depressing driving experience, and the higher line models approach $20k which is just plain stupid for one of those.