Being post-legged puts a lot of strain on the stifles and yes, could break him down much faster than a more conformationally correct animal. Also, I have noted that post-legged horses are awfully uncomfortable to ride in the cart with (the cart bumps up and down a lot) as I drove a couple Belgian draft horses that were horribly straight in the stifles. It was not a pleasureable ride.
Are you mostly walking these miles? Walking will not be as detrimental or uncomfortable for you or the pony. Trotting will be the much rougher gait for both of you.
Joint supplements don't neccessarily prevent a problem, it just eases it once it's begun (at least my vet recommended that for a horse that had an injury but no arthritis in the joint yet) so save your money on the joint supplements until the horse has an issue.
I don't think boots would help this issue much either.
Only you know what is a reasonable price for this type of animal in your area. Horses are so individual, that is why the price varies and is negotiable most of the time.
Don't know if that helps you any.
Andrea