If you still don’t pay, the tax buyer will foreclose and own the property. 

That's exactly what I did when faced with this issue, meaning I had a buyer under contract before I actually closed on my purchase.

I'm going to be honest so I can paint the picture as to where I am as a new investor: I've marketed for buyers, practiced calculating ARV numbers so see how that works, and have searched for properties. 

He says he already has a few buyers interested in buying it from him, as is, for substantially more the we are getting it for.

I know of one on Long Island that I use on the regular for my traditional buyer and seller listings, who is also skilled in double closings.

Is it out of the ordinary to ask to do a walk through with a contractor so that I can bring some concrete numbers to my buyers?

in my mind this is something to flip to someone who can actually close it and operate it.. and I am surprised an owner got this far with out vetting the OP as a real buyer.. 

I'm not saying it isn't a good deal, I'm saying how many potential buyers can fund a deal like that?

Just sell it with a quitclaim deed or do owner financing with your buyer.

For a decent quality note with a decent yield, there are a multitude of buyers at any point in time.