Welcome to the site nervekiler.
I had a 51 a few years back with a 350/400 combo. These AD pickups as they are known (Advanced Design) make great rods. Everything is available for them from minor trim to the big stuff. Lots of aftermarket stuff for hot rodding as well. If you like the truck and it is everything you want then $4500 is an OK price. I think you can get a little better truck for less and a whole lot better truck for a little bit more. I don't think from the looks of that truck I would be selling my daily driver (Camaro) before I put quite a few miles on it and checked it out from the bottom up.
There are several members building AD pickups, suburbans and panels on this site. Check out the Project Journals section of the site for the members builds. As well use the Knowledge Base and the search function for quick answers to your questions. Here are a few links I have collected over the years:
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