Here's my answer to your questions in order:
1. Yes, bite the bullet and go for new ones because of safety and your truck is so nice it deserves no less.
2. Yes you can find a few used but why? See #1.
3. You can find peopke who will work on your wheels if you check around. In OK there should be many small town tire shops and farm/ranch supply places that can help you out.
4. You claim to want to keep it "as original as possible"...so why would you put larger tires on it. They simply are not needed for the small GVWR of this chassis. Probably not as heavy as an F450 and you've mentioned the money issue in new tire remarks, so again, why spend more on larger tires that may not fit and serve no purpose?
From all you stated my opinion is that you need to decide upon a course of action before starting this project. It is your truck so these opinions (remember you asked) don't mean anything but get yourself organized and a plan together is pretty good advice even if it does come from me. Good Luck and again it really is a very nice truck so your way ahead of many already.
