One important piece of advice when using the spot welder once you buy one is to perform a peel test before using it on the car.
Get two 2"x1" pieces of clean steel the same gauge as the panels you wish to weld and then spot weld them together at one end.
Then with a couple of pairs of mole grips clamped to the other end if each piece, try to tear the two pieces apart.
If the weld let's go and the pieces come cleanly apart, then your weld is not successful.
If the steel tears apart leaving actual weld intact and a hole in one piece, your weld is successfull.
Not sure how useful that test will be on this sort of spot welder where there may not be any power settings but if there are any you can adjust to suit and have it set correctly.
Similarly, if the weld blows a hole when you make it, the power is too high and needs to come down.
I do this before each welding session as there is nothing worse than finishing spot welding on a panel and then hearing the random pings of spot welds letting go
I can't really recommend a make or model having only ever used industrial 3 phase welders but good luck.