What gears are in the V10 would be my deciding factor straight up. With 35's if she doesn't have 4.56's MINIMUM for the weight you are looking to pull...my opinion is that you would be better served with the PSD.
I pull 9000#'s with my 4.30 geared stock tired V10 Ex...it is a beast of a pull dog. Tall tires reduce effective final drive ratio. If that Ex has 3.73's in the rear end to start...your 'effective' final drive will be in the 3.41 range...NOT a good gear for towing...let alone towing 10,000#'s with a V10...and I'm a V10 guy!!!
Plus depending on how the guy did the lift, what load range tires he used, etc...you may find that finding the towing 'handle' on that Ex especially with the weight you are towing somewhat challenging...very doable but challenging is all I'm saying...and that would hold whether it was a V10 OR PSD...lifted vehicles can be a handle to find the handle on is all...
As far as your other questions on tranny...ALL 7.3L and V10 Ex's have the SAME tranny as your F350 you just got rid of and all are in similar mileage range as your 01...so that might be a crap shoot...did you only get rid of your F350 because of the tranny issue?...I'd hate to see you buy a new used rig and have the same issue bite you in the butt...
The 00 PSD is a time proven motor...I'd see if you could work the price a little (is it a private sale or car lot?)...either way I'd see if you could negotiate in the price of an extended warranty 'just in case' the tranny gives up the ghost.
Also Ex's (and all SD's for that matter) are prone to ball joint issues...if these are still stock ball joints...those may be on their way out also...those two jobs (tranny and ball joints) could easily set you back $3000 or more to fix...
If it were me...given the weight you are talking about pulling...unless you can negotiate in the price of new gears for that V10 and get him down a lot more on the price...I'd personally go with the PSD but also work him on that price too...
Good luck...decisions are NEVER easy but hopefully you will get some good things to think about from the group in here...
Let us know how it goes!
And WELCOME to FTE!!!
Joe.