My book isn,t the latest on the HP but it says the D6 was 80 HP at 9U 16,5xx but if I recall the last of the U models were about 90 HP I believe and yours would be in the middle to later 50s before they went out of production as my book only shows starting up year.I have no specs on the weight of it.

I show it as being a 1957 model.
Here are some specs and weights but the hp did grow in the later serial numbers with production ending at 29764 (1959)

A 9U with good/usable undercarriage would probably bring that if it runs and operates correctly otherwise. One with worn out undercarriage would probably not bring that around here (more like scrap value-less hauling costs in that case)

A lot of the value depends on the undercarriage life left and perhaps on the equipment it comes with (dozer, ccu, hydraulic unit, brush cage, belly pan, tool bar etc.)

I would look at the hours on the service meter, but don't put a lot of stock in the reading since they broke down pretty regularly and could have been replaced at any time or several times in the life of the tractor. I would put more stock in the general appearance of the tractor, the state of repair/maintenance it is in, and a test drive, than the hours.

Some of these old tractors haven't had any maintenance because they weren't "worth it" any longer and then some folks never did fix things even when these were newer.