Depending on your climate, I'd want that spa filled, running, and heated before I bought it. Even if they offer to write a contract to fix it, you'd still potentially be moving a spa to your house and setting it up, wiring it, filling it... only to find out they didn't winterize it and have a leak. When buying privately, I prefer to see the spa in operation to make sure it heats, runs, and their patio isn't wet from any leaks.
Price wise though, that sounds like a pretty good deal. The Hot Spring Vista is a great spa, and honestly it sells for more than double that in my market brand new.
As far as the concrete pad is concerned. Hot Spring actually recommends a 4" reinforced concrete pad for that spa. See page 4 of the following document:
2007 Hot Spring Pre-Delivery Instructions (Foundation, Dimensions, & electrical wiring)
http://www.hotspring.com/pdfs/pre_delivery...ctions_2007.pdf
You can find the 2007 Hot Spring Owner's Manual here:
http://www.hotspring.com/Hot_Tub_Customers...e-delivery.html
The tub is in south Mississippi so I can't imagine it having any leaks/problems from frozen pipes etc. It only freezes a couple times a year and rarely longer than an hour or so when it does. I will request to see it running and heated.