If you are doing this for the auction & are "unsure" what you might buy I would say get their however you planned to get their and don't even worry about renting a truck. See what you can buy - then rent a van and a storage unit. Move the machines you bought to storage then make the trip to go pick them up sometime down the road.
I know this sounds like a "crazy" approach but in the end you will save money because you will only buy stuff at logical prices since you are not trying to "fill a truck". Who knows how crazy prices might get! And on the "flip side" if things are going cheaper than expected you can always rent additional storage units and make more than one trip to pick machines up.
I have done things this way more than once and it has always worked out great for me. I never felt any pressure to "overpay" just to fill up a bigger vehicle, was able to move stuff home at a time that worked out great for me and in the end it worked out being "cheaper" that way every time.
Two years in a row I even rented storage near Allentown PA a month before the Allentown Pinball Show just to "stage" machines in Allentown getting ready for the show. Then moved the machines from storage with my van & trailer on set up day.
Another thought if you don't mind towing a trailer is rent a smaller truck & a trailer to pull behind it. That way you can load 8 machines into the 6x12 trailer and don't have the hassle of getting everything up the ramp into the rental truck.