I have a 1953 Ford F100. What size electric motor would be OK, and what type and price range?
First, I love these old dudes as much as the next person. Don't get me wrong. But it's not what I would choose to convert.
Why? It's too old (parts? rust? heaviness?) and too heavy (because back in the '50s when it was built, we were swimming in cheap petroleum and there was no reason to build cars light or aerodynamic).
If you're still determined to electrify that grand old man, and I can't blame you, then you'll be putting a lot of batteries in the back. As for the motor, how about putting a pair of NetGain WarP 9"s in it, like John Lussmyer did in his electric F-250? Those are series wound DC motors, and they cost $1750 USD apiece in 2009.