What Kevin points out above is a possibility you always have to consider. You may not have to change preamp tubes, but you may find yourself having to change power tubes not too long after the purchase if the owner has never changed them or has used the amp a lot with those tubes.
Regarding your needs, I'd say just snatch that Mark IV up! I was thinking about suggesting the Mark III, but you definitely won't find a good clean, crunch and high gain at the same time in that amp without changing the settings. While I am able to footswitch between a clean and a high gain sound that are both superb for my needs, I know there are many people that haven't been able to achieve that and view it as a compromise.
At the end of the day, it's always best if you can try the amp first. That's what happened to me, as I was always hearing how you "couldn't get both a great clean sound and a crushing high gain sound at the same time from the Mark III because it shares its' controls". However, I found one in a music store, just laying there, forgotten, and was able to try it out. Having read the manual previously, I knew exactly how the controls worked, and I was able to get both a superb clean and an awesome high gain sound (for my taste at least) at the same time! If I hadn't been able to try it out, I would probably have never considered it as an option.
The other alternative is to find a used Mark V for even more flexibility, but I'd snatch that Mark IV for that price and then try a Mark V down the road.
Sorry for rambling too much and best of luck!