I have a lot of panels to stitch together, drilling, welding, grinding, filling etc etc isn't much fun! Just done a rear valance with the mig and it's taken ages to get nice
Keep on looking for a proper spot welder, eventually one will come up. Don't worry if it looks old and knackered, they are basically just a massive transformer so not much to go wrong.
Also no need for a timer on it and just a basic set of 8" arms will do almost everything you need on a mini.
You will be amazed at how quick they are to use, also all the spots will look like factory welds.
After my metal shears it's one of the best tools I've got hold of, has loads of uses outside car constructions as well.
Only downside is the power requirement, it needs a decent supply and might blow the trip on a normal domestic ring main if it's protected by a 16a mcb.