What to watch out for when looking for 69-72 4x4 Blazers / Jimmys

[71elect]

Hello everyone!

I've started watching for the 69-72 4x4 Blazer \ Jimmys that are for sale once in a while in my area and i've determined that I need some advice on what to be aware of. I've owned the 2wd & 4wd pickups but never a Blazer \ Jimmy. What problem areas should a guy be watching out for when out searching for a 69-72 4x4 rig? I've found on this forum that there are 2 types of hard tops (single wall & double wall), how do ya tell the difference & which is more desireable? Any problem areas on the fiberglass tops that need to be watched out for? What about the doors, any way to tell if it's a hacked down pickup door? Are the windseild frames prone to rust out easily? What areas of the tub need to be watched for rust? Roll bars, did they come from the factory as an option or standard equipment?
Sorry for all the questions, but I would rather have knowledge up front to know when to buy & when to walk away.

Any advice you can give will be greatly appreciated,
Thanks,
Josh