It's the 15 year old part of the equation is part of what makes me think that the price is too high.
Value starts to fall off gradually at 12; the only exception to that rule would be a horse with other skills, a show record, etc, you might pay that for a 15 year old. If you're south of Richmond, VA, for what you describe, $1500 - 2500 is a more typical price for a safe, broke, trail and pleasure horse you can continue to learn on. . My riding buddy bought a horse last year, same area, and essentially what you're describing, grade horse, aged, totally bomb proof confidence builder and packer and paid $2000.
I do like him, based on that photo, and I think draft/QH is a lovely cross for what you want to do.
I agree with all the other advice given on this thread.
And if you do decide to purchase him, DO have a pre-purchase exam done by your own vet while you are present.
$3700. might not be too much to pay for your perfect horse that you'll enjoy for years to come, but it's definitely too much to pay for one that's older or unsound.