If yuo do decide to lease him, make sure you have a very well put together lease contract. It is useful to write one up and go to a lawyer to have them firm it up and clarify it so there are no holes.
Personally, I wouldn't expect the horse to go past training level at a competitive level. From a training perspective, the horse could be performing movements from PSG if he is talented enough, but he will probably never be able to put a test together well enough to be competitive.
To put this in perspective, I owned a quarter horse, in training level at our best we could score 65%, in first level at our best, we had trouble breaking 60%. At his peak he was schooling half pass, shoulder in, travers, changes, and starting pirouettes. Even though he was schooling these movements and doing them quite well, in a competitive test setting he would not have scored above 55% past first level because he never had a medium or extended trot and his collected work was difficult for him conformationally. Keep in mind that these days in dressage to be competitive at any level you need to be consistently scoring in the high 60's and low 70's.
I would be wary about how sound he is, a 7 year old horse should be able to canter. Before taking any further steps, I would ask for a complete veterinary exam.