welcome to the club.
Your gonna find that 60-66 burban parts are fairly difficult to find. A great deal are simply not made or reproduced anymore.
Now from the doors forward, the burbs share nearly all parts with the pickups. But from behind the door back, your gonna have to search for parts trucks, good pick your part locations, craigslist, evilbay, and any other resource you can find.
Engine and trans: the options are just about limitless depending on your skill and budget. A SBC is always a good choice and with few exceptions probably the easiest to put in. Or you could do like me and completely loose your mind and try something as absurd as to attempt a Cummins Diesel. I dont particularly recommend the cummins unless you enjoy pulling out your hair and are a masochist.
center seat: Good luck. You can still find them but they seem to be harder and harder to find. And when you do find them, they seem to be either $$$$ or POS. But they are there, you just have to keep an eye open.
Trim: I dont think I've ever seen them for sale. Oh, I see people looking for it but I dont think I've ever seen anyone find any except for those that had it made special. Or got lucky and found bits and pieces off parts trucks.
Windshield: all 60-63 pickups and burbs share same windshield. I've heard that safelite does have them available. Dont know if it is true or not but worth a shot.
I'd recommend getting catalogs from LMC, Brothers, and truck stop in orange. And if you are in doubt about anything in particular they guys here are an absolute treasure of information.