S100,
I have owned Mark II, III and 5 guitars in the past. I currently own and use a '75 Mark 4P guitar, with a recently installed Barbera Soloist pickup in it. Basically, the III is the middle-of-the-middle of the line of Guild Nylon Mark guitars. The Mark 4 is an upgrade of sorts to the III (ebony board, padauk or pear wood in place of mahogany), while the Mark II is the matte lacquer version of the III.
The Mark 5 sounded great to people in front of it, but I could never capture the sound I heard out front when I sat on the back side of the guitar.
I never owned any of the more expensive Mark guitars- VI, Mark VI special or the Mark VII, but I owned an early-mid '70s Ramirez 1A and I think I understand high dollar nylon classicals.
So that makes me the perfect person to go out and tell you to buy a Mark III or Mark II and have a blast!
Let me warn you about 3 things:
1) If the tuners go bad, it's hard to get a set that fits. They are a special size. Not impossible, hard.
2) Beware of high actions. My Mark 5 had a very high action and I ultimately sold it. Come to think of it, the II and III actions were too high for me as well. While it is true that I like low action height on nylon string guitars, I firmly believe that Guild didn't pay much attention to the issue of action height. It simply wasn't part of the equation.
3) If you see a crack on the top, it may be hard to fix. The traditional classical bracing on a Mark guitar makes it hard to get to the crack from underneath.
One guitar I owned had a crack right next to a bass-side brace. Some former repairman had put a large piece of wood right next to the underside of the crack. It was wedged and glued in so tight that we couldn't get the darn thing out without popping the back!
The Mark 5 had an crack right along the middle seam of the top. It was directly on top of the center top brace, so we couldn't get to that one either.
So the moral is, don't buy something with cracks on the top and don't buy something you can't return if the action is too high.
Luck to you, gil