LMC isn't the preferred vendor around here but their catalog has some very usefull diagrams. Definately sign up for one.
As far as tops go, the single wall is more desireable to most people. It bolt down from above and where as the double wall bolts up from the bottom. The hatches are the same on both. A lot of people, including myself, have soft tops.
http://www.specialtytopco.com/ and
http://bestop.com/ both make tops for first generation Blazers. I have had my truck about 2 years and have yet to put the top on it.
The grills are different between the GMC Jimmy and the Chevy Blazer. The quickest way to tell is the Jimmy has four headlights. Parts are more readily available for the Blazers.
I looked at a blazer for sale at the Public Auto Auction in Woodinville, WA (it is really just a car lot). It had at least 38" mudders, a lot of lift, a bright red paint job with a soft top. Very nice looking, from a distance that is. The sales man said they wanted $15,000 for it. One look from 5 feet away and I knew they were high. The body was wavy, the gap betweent the fenders and door was over an inch at the bottom an snug at the top and then I opened the door. There was rust on the floor boards leading down to the inner rocker panel. I looked under it and the rocker boxes were all rusted out. I told the salesman that this was maybe a $2000 Blazer. I think he agreed but I'm sure he won't tell the next guy who comes to look at it. Some one is going to get ripped off,
just don't let your friends buy this truck.