I just installed a brand new digital control system, with a 5KW 220v electric heater on a spa which was obtained under similar circumstances. There was no place on the shell to put the topside control, so I had to mount it in the deck next to the tub, which works fine but they have to keep it covered with something. But let me give you a little history on this "free" tub:
I replaced the two-speed pump a month after they first got the tub from their neighbor. That little exercise was around $400, and it was only that low because I had a good used pump/motor combo which I was able to freshen up with new seal and o-ring. It is 50' from the Pacific Ocean, so there wasn't much left of the old pump motor...
With the tub running on a temporary 110 power cord, they tried it and liked it, so they decided to proceed with having it hardwired 220volt. They could not figure out how to make the swap, after paying an electrician $800 to run the power, so I made another service call and got it switched over and running. $125.
It ran for a month, then the heater fried. I had to order a complete heater with header tube - it did not have a removable element. I figured the pressure switch had failed, burning out the heater, so with a new heater assy., new pressure switch and labor, they were out another $400.
It ran for a month - then the GFI breaker started to trip. The heater tested ok, the blower was dry - it just didn't want to run anymore. The customer OK'd the installation of the new box and
one thousand dollars later, they now have a digital control system which works great. That paid for a very basic box with the Balboa M7 system. It only has to run the heat on low-speed, the jets on high speed, a blower and a 12v light.
Side notes:
The tub is poorly insulated - about 1/2" foam sprayed on the back of the shell, very open cabinet.
The tub needs a new cover - $400 waiting for the approval.
The jets do not draw air. I have not figured out yet if the new-looking jet nozzles are the wrong type (I suspect) or if the whole tub was plumbed wrong when they moved it over the fence and hooked it back up. Water flows out of the air control unless you tighten it down, and the jets are mellow, to say the least.
The tub has four jets. Total.
They are getting close to the price of a brand-new Freeflow spa, and with the cost of heating it, I predict they could pay for a HotSpring Limelight tub before long...
HTH