If you happen to be the handyman type,most repairs if and when they arise,you could handle on your own. So in that regards,looking at a used tub with zero warranty,is a route you could persue.
In the 8 years of ownership of my spa,I have fixed various things,and found parts via online.
Ultimately,I would suggest going with a name brand,as they are built to a higher quality compared to Ebay tubs etc. You'll find they last longer,have high quality parts,and can find parts for em etc.
FYI.....I have had zero issues with my "Caldera" shell,which is now over 8 years old. When I initially purchased the spa,1 of the things that impressed me with the spa,and Calderas product line,was the way they manufactuer the shells. When you go look at spas etc......where the lip of the shell overlaps the siding of the tub....try taking a look at that. You'll have to bend down and look up,or,turn your head upside down and look underneath lol. Visually looking at the edge of the shell,will give you an idea of how it's made I.E. layers involved,thicknesss etc etc.
If any other questions,don't hesitate to ask.