Check with your local spa dealer to make sure they got parts before anything else. If you can, give it a test run before you move it. Since it sounds like it's been sitting outside for years?, and you know rain has gotten in. Inspect it first for any potential rodent damage in the insulation, any problems with the running bits. If that's good, give it the fill test. Fill it, and let it sit for a couple days, make sure there are no leaks. After that, plug it in and run it for a while, make sure it's not leaking and the controls are working. If all is well and working, then you can drain it and move it to your house.

No way to run it where it is. It was never hooked up at my parents as they didn't want to upgrade the electrical for it. It's been sitting outside for around 15 years.

Then you can inspect it and do a fill test and then if that passes move it to test run it. Outside for 15 years isn't a good thing for most things to sit.