I knew most of the players at one time, but that was some years ago. I had the least ties to Mecum, don't have anything good or bad to say about them. If I knew who was auctioneering/working for them now I'd have better advice.
In the classic car auction market you have some guys who are pretty honest, some who are complete crooks, and a bunch who are in that gray area in the middle. Some owners will talk to potential buyers, some won't want to have direct contact, and the auctioneer won't typically put you together with them. Happens sometimes, not often.
Most don't certify anything, it's as is where is, like most auctions, so bid accordingly. Some cars will be different, haven't worked personally with Mecum so I can't say what their policy is now. Per your link I saw no warranties and if you don't see them they probably don't have them. That part in their terms about your decision being based "solely on the buyer's inspection" should be taken to heart.
That isn't a bad thing or a deal breaker, that's pretty much how auctions work, esp. classic car auctions. The more general car auctions will sometimes sell under a "light system" where a green light indicates it's ride and drive, red means it won't start, etc. These high end classic car auctions usually don't use that system. They'll announce about the car but they don't warranty a lot usually, esp. with custom jobs like this one.
Wish I could provide more specifics on Mecum but I only knew those guys in passing at best. Dan Kruse/Leake still work the Dallas area I believe, I do know those guys.
The best advice I can give is
1) Inspect the thing fully. Don't assume anything, do your homework.
2) Stick to your price, be reserved in your enthusiasm around everyone there.
Know what you're buying and keep your cards close to your vest and you'll be fine.
