The only thing I've really heard of to "stay away from" is concerning engines on some of the later lawn tractors. Something about the oil delivery on inclines.
But any K series engine on an IH Cub will treat you well. Otherwise, I like the wide frames with their quick attach implements. I have a 149 and 1200. Personally, I like the look of the QL the most.
One thing is for sure, whatever you get, you'll get all the answers you need for it here.
I've got an old Gravely walk behind my dad used for years sitting under my porch. It's next on my list, as I really need a walk behind brush hog...
__________________
-Jason
Cub Cadets: 1200 | 149 | 1650 | 122 | Z-Force S 54
Implements: QA42A | Push Blades | #2 Tiller | Rear Blade | 223A Utility Trailer | And a few mower decks
Gravelys 5260 | CI Walk-behind (Project)
And a couple of Subarus
