I just picked up a used 440 gallon spa and have a few questions. I intend to decontaminate it, so I ordered a bottle of Spa Flush & new filters. First I need to fill the spa to test out the motors, blower, heater, check for leaks, etc. So once I know everything is good:
1. Should I bother with balancing the water? or can I just pour in the spa flush and follow directions?
2. It may sit with water for 2~3 days before the spa flush arrives (couldn't find it locally), should I be adding bleach for those few days?
3. Once the spa flush is complete I planned to do a 2nd flush to be on the safe side, can this be done with cold water?
4. After decontamination and I'm ready to start-up, can I balance the COLD water? Or should I wait until it's heated?

FYI, I plan to use the Chlorine method outlined here on TFP, and I've purchased a new TF100 test kit and Salt test strips. Once the water is completely balanced I would like to add salt to the water to around 3000ppm using the salt recommended on this site for adding to swimming pools. Will this cause any issues being that I'm adding it to a hot tub rather than a pool? Are there any water chemistry or other issues I should watch out for?

Thanks in advance for any help!

You don't need to balance the water when using Spa System Flush except for whatever it requires in its directions.

As for the few days before the decontamination, adding some bleach wouldn't hurt, but it may get consumed fairly quickly. At least it should slow things down from getting any worse. Don't add too much since the bleach won't be moderated in its strength by CYA -- add no more than 10 ppm, though even 5 ppm FC should be fine if it lasts.

If you want to do a second drain/refill you could certainly do so using cold water, especially if your intention is just to more thoroughly rinse out the spa as opposed to doing anything chemically to it where higher temperature is more reactive.

For your final fill where you will be balancing the water, you don't have to wait until the water is hot to balance it except possibly for the pH since pH will naturally rise with hotter water, all else equal. You can just target a higher pH level if you are balancing the water cold -- say 7.7 or 7.8. If you balanced to 7.5 and then heated the water, the pH could get lower. Going from 50ºF to 104ºF would have the pH drop around 0.3 units (though carbon dioxide outgassing will tend to make the pH rise so might obscure this effect).

You should check with your spa manuals and manufacturer's recommendations before you add salt to the water. Unlike pools, spa components aren't necessarily as salt tolerant and some spa manufacturers void warranties if you use high salt levels. The higher conductivity of the water can lead to faster metal corrosion unless higher quality stainless steel components are used. The hotter spa water accelerates chemical reactions including those involving corrosion.

Thanks so much for your advice. I was able to "quickly" dry test both speeds on both pumps, the light, and blower. So I may just wait for the spa flush to arrive before I fill, leak test, and flush. I guess I'll find out if the heater works at that time.

On the salt question, this is a Craigslist find and not much is known about the spa. Apparently the brand is Superior Spas out of Okc, but they went out of business a few years back. So warranty is not on the table anyway. I was lucky enough to find out the molds were sold to Sunbelt, and my exact spa model is still being sold through Stellar Spas, so I can still buy replacement pillows and such. That being said, the spa pack/heater is a "Spa Builders M-Class" which seems to be a fairly high end pack. Do you think that equipment would have an issue with the salt content? I assume that the heater is the only concern since everything else is plastic and shouldn't really be affected by salt.

Oh one more thing, according to website that sells the same model I got, it's supposed to be 440 gallons. Is that usually pretty accurate or would I benefit from watching the meter to determine actual gallons?

This is the tub I got, $350 for the tub, $200 to haul it to the house, and $5 for the blown fuse that caused the previous owner to sell it (he thought he needed a new controller). Seems like a pretty good deal, just need new pillows and a new cover to be complete.
http://www.stellarspas.com/sanjuan_detail.html