Personally, I would not even consider replumbing and reskirting a spa that is that old and not a name brand. That is a pretty big job and there is quite a bit of potential for problems. You would be much better off looking for a refurbished spa or spending just a little bit more and getting a new spa.
You mention that you like the fact that it heats by pump friction. I'm not sure that I understand why this is important to you. I would personally prefer a spa with an actual heater in it. If energy efficiency is your concern, you can still achieve that with a normal heater. Spending all of that time and money to refurbish a spa that would still have 7 year old components and an even older shell just does not make sense.
I wish you the best of luck whichever way you go
