Well my 83 has no sunroof, but that doesn't mean every other 83 wont. Hmmm the delema on how to find out ......
On the head gasket question chances are that if you need to ask then you wont be doing it yourself so will cost quite a bit. However, you may notice that no one who owns one of these sticks with the original engine because they quite simply suck. If you are going for a conversion however you may be on a winner ....
It all comes down to the $ you want to spend to buy the car and what you can do yourself.
I know I haven't answered your questions but they aren't questions that can be answered really.