1. You dont need o-ringed heads unless your going to be running a good amount of boost... stick to non o-ringed heads,
2. Limited slip is better for street cars, locking is better for racing applications. You'll want the limited slip because when you go around corners, you'll want your outside wheel spinning faster... locker both wheels will spin the same speed and will cause a lot of problems in wet conditions
IMO, O-ringing is a waste. Now, I do have very old school views from time to time, and this is one of those times. If you are worried about blowing headgaskets and what not, have the block decked and use a good gasket. I'm sure I'll get ripped for this...
The heads on my coupe were o-ringed, and unless I was doing something wrong when I would pull the heads the o-ring in the gasket never seemed to be exactly in the groves in the head. just my .02 worth.
It will not be that expensive to do a single turbo setup. I am not planning to do it for a little while anyway. How Much $$$$ do think it is going to cost?