Unfortunately I have failed to get too much information about Humvees from the usual sources. Were there many changes from when they debuted to modern times? And if so, what was available in the 1980's as opposed to nowadays?

Your question is almost impossible to answer in a post here. In short the United States HMMWV evolution vaguely breaks down like this:

-Legacy HMMWV to include M998, M1038, M1025, M996, etc.
-M1097 which is an improved M998
-M1114 Interim Armament Carrier
-Most modern versions are M1151 Interim Armament Carrier, M1152, M1165, M1167 and M997A3.

This is a very vague synopsis. I could bury you in details but I don't have time to type it all. You can visit my website that has links to TM's that have detailed information. www.hmmwvinscale.com

Here are two other great websites:

www.humvee-models.com

www.warwheels.com

There is a fairly decent summary of vehicle changes here:

http://www.globalsecurity.org/military/systems/ground/hmmwv.htm

Broadly speaking, there have been three generations of Humvees, and the external and internal changes would really take a small book to denote.
It's easier to just look at kits. The oldest, 1980's-vintage kits, from Italeri, Esci, and Tamiya (which used mostly Italeri parts with a Tamiya gun mount), were pretty weak, especially the tires. Esci's body shape was also a bit squashed in front, looking more like a Volkswagen than a Humvee, but these are long gone for the most part.
The Italeri kits are still around with modest improvements and body style updates (sometimes in Revell Germany boxes), but the Academy kits are superior (and inexpensive). For first generation Humvees, the Academy kits are generally satisfactory representations, if you want to model the 1991 Gulf War, or early stages of the Iraq War. They have vinyl tires, which look better than the styrene tires in the older kits, but there are nicer resin tires out there too.
Tamiya's more recent Humvees, which came out just as the Iraq War was starting, are good representations of the mid-production types, and the recent Bronco kit is a very nice interpretation of current variants.
As you might expect for a vehicle that has served for over thirty years, there are numerous resin and photo etch updates and conversions available, and you can spend years just building Humvees, if you want.