I'm using a 1 farad Rockford fosgate capacitor that I paid $31.51 for on fleebay about 5 years ago. One of the threads on the unit I bought was goobered, but I just drilled and tapped it for the next size up and all has been fine ever since.

This unit looks very similar to the one I purchased. Ideally look for an older unit that's purely passive. I see a number of cheaper used options available from other manufacturers, but I am not in a position to be able to recommend any of them.

Using a capacitor seems like it would be cheaper than using a lead acid battery unless you already have a spare battery on hand. As an added bonus, capacitors are pretty shelf stable so you won't have to worry about caring for it when you're not using it.

The voltage needed for spot welding depends on the thickness of your nickel strip, the thickness of whatever you're welding to, and the overall resistance of your spot welding setup. Somewhere between 7 and 8 volts was enough for what I was doing. Start low and work up, you don't want to blow a hole in a battery cell.

I'm not sure that a capacitor could jump start a car. That takes a stupid amount of current to get the motor turning over, something like 200 amps iirc. Then you need to keep the engine turning long enough to fire. A 1 farad cap charged to 14.4 volts stores 103.68 joules of energy. That sounds like a lot, but when you convert it to watt hours it's only 0.029 watt-hours. So unlikely to start a car, but it sure can deliver a metric ton of current for a very short duration without having to worry about the longevity of the cap.