I don't have any experience, but I believe the Elna SU air electronics are collectible machines and highly prized by those who have one.
If I am reading your post correctly, the total cost would be $400. Could you perhaps dicker with the owner, pointing out that it would cost you $200 just to make the trip? If you could get it for $300 total, it might be worth it.
A lot depends on what your taste is. If you love older machines and cabinets, this could be a great deal.
Concerning the walking foot, I'm sure there are walking foots available for the Elna. It is probably a standard short-shank machine. The Janome foot may have a different shank, though.
Harp size becomes important when you go to machine quilt your tops. With a small harp size, you need to split your batting in order to machine quilt a large quilt. However, it's do-able provided the motor on the machine can handle quilting.