What is the best possible 80's coupe deville to have to convert over to the early 90's brougham look and why? Also, what is the best year in the 80's coupe deville to have and why? Would it be the '80 with the 6.0 litre?

One that doesn't have a 4.1L V8 in it!!!

Get a 7L or 6L car would be my first thoughts. Or get a 5L Olds powered car. They are pretty solid and can accept any larger engine with ease and are easy on gas as is. They 5L Olds powered cars have the best potential for best gas mileage (if that is a concern, they are rumored in Milwaukee to be at $2.899 a gal for unleaded by midnight tonight, it is a concern for me!)

Didn't the 81-85 Coupe Devilles only come with the 4.1? I know the '80 had the 6 litre but after 80 for the Coupes it was only 4.1 I thought.

The best coupe to convert is a Fleetwood 2 door. This is not the same car as a Coupe Deville and a lot of people (including myself for a long time) do not know they even exist. 1980 with a carbonated 368 if you hate computer controls. 1981 if you appreciate the 8/6/4 digital fuel injection. The Fleetwood 2 door was made all the way up to 1985 model year (1 year after the RWD coupe Deville was discontinued). After '81 you will find mostly HT4100 cars (mixed in with V6's and IIRC diesels) which will break sooner or later unless you drive like a 105 year old. These cars can convert over to older or newer drivetrains.
2nd best car is a plain Coupe Deville. Everything same except no 1985 RWD.

Here is a picture of one for sale....
http://cgi.ebay.com/ebaymotors/Cadillac-Fleetwood-1983-CADILLAC-FLEETWOOD-BROUGHAM-COUPE-RARE-SUNROOF_W0QQcmdZViewItemQQcategoryZ6149QQitemZ4568 891239QQrdZ1QQsspagenameZWDVW

That one on ebay is one sweet ride.

Here's another:

http://cgi.ebay.com/ebaymotors/Cadillac-Fleetwood-Coupe-Outstanding-Top-Quality-Driver_W0QQcmdZViewItemQQcategoryZ6149QQitemZ45692 97782QQrdZ1