Definitely give that Mark I a try if you can. Worth a good look. I tried a vintage Mark I recently and was very impressed. As you note, the tamer channel definitely gives some tones that are worthy of the Fender blackface amps that inspired it. But the other cool thing I found, in an admittedly brief try, was this:
The hotter Mesa channel on the Mark I can itself yield some great full, fat cleans. The kind of cleans where you can hear the power of the increased saturation in them, but clipping isn't quite there yet. Of course, you can crank the volumes to get serious gain, but dialed back, that Mesa channel could, me thinks, push out some great liquidy Jerry lead tones.
Final note: I've had and liked the Mesa Mark III. But I like the Mark I better. Not as versatile, but in my brief test, sounded closer to it's Fender heritage than the later Mark III. A simpler, purer tube tone. My unschooled assumption on this is that as the Mesas went along in development, more versatility was added, but some of that guitar-straight-to-tube tone was lost... Hidden by more complex circuitry. Going by my ears here... Not by any technical knowledge at all... So take it with a canister of salt!
Try out that Mark I. You'll like it.