I think Clarion and Pioneer make those size radios more than most others. I'm not familiar with the newer Mountaineers, but I think you're talking about the 1.5 sized din? GMs are like that, so it's a large enough market to make them. If it's double din, there are some too, but I think it's going to be a little more money, because manufacturers will cram more features in them.
After wasting lots of money, and often overbuying, this is the conclusion on the order of buying things I've come to:
Speakers first. The sound difference will be immediate and ovbious once installed. I bought my radio first, but there's little difference if you have the same speakers sometimes.
Headunit second. If you have the Alpine changer, get an Alpine head unit that directly connects. A direct connection to the headunit outside of the antennae often will relay more range and frequencies with better sound transfer. New head unit will require a new harness, about 15 bucks, and new wiring harness, about another 15 bucks. All they do is take out your old radio, put the new one in, connect the new radio wires to the radio harness wire, which has a perfect fit plug into the sound system. High RMS radios may cause the stock amp to clip, (I had a Sony, and it clipped with 15 continuous watts) but now have an Alpine with much lower watts (7-12) and it does fine directly connected to the stock amp.
Finally, if you want more power, then go for the amp. Don't let those salespeople tell you what you need. The wiring is really not difficult that you need to buy a new amp and scrap the old one. Just look at what was written above. For one thing, they're salespeople, and sometimes, depending where you go, they are not the installers, just salespeople. It may sound better, depending on what you buy. They only tell you the wiring is difficult, and they'll say that because your factory head does not have RCA preouts. However, Audiolink sells a converter for under 40 dollars that can convert a speaker wire out to the an RCA connection.
You don't need to replace an amp just because you buy a new radio. New don't need to buy a new radio, just because you get a new amp.
Just trying to make sure you don't get wool pulled over your eyes. Good luck, Jon