I thought it was worth it to pay extra for the bus I wanted
We paid $7k - of course I wish I had been the one to snag it at the auction the summer before, and save myself $3k - but that didn't happen. But I love the bus we got - has everything we wanted, and it's a unique model to boot! Only met one other person online who's got one.
I think it all comes down to what it's worth to YOU. If you have something specific in mind, and are willing to pay a bit extra to get it, then don't worry about it. Especially if you're going to put $15-20k into a conversion, spending a couple thousand extra to get the bus you want to start with makes sense.
All that being said, my purchase from a private party was less than smooth (he didn't actually have the title)... took months to get it all worked out—so be careful there.