You could buy a spot welder, but you wouldn't have much use for it afterwards.

You could have a shop do a number of spot welds for you without approaching the cost of a bargain welder.

Harbor Freight has them for cheaper, if you don't mind their overall level of quality.

There isn't that much to go wrong with a spot welder. It's a (stepdown) transformer with two copper rods. You can pretty much buy a used one without much worry. Since they don't get used much, it is doubtful that the used one even has much welding time on it anyway. I would still go with a name brand siply because the quality is usually better.

Plug welds (or more properly rosette welds) are fairy easy to touch up with a grinder. They are usually stronger than a spot weld. There is no reason that you can't hide rosette welds just as easily as a spot weld, USING A PROPER WELDER. Welds done with better equipment are almost always better than those done with bargain basement equipment. Rosette welds done on a cheap machine will stick up, pit excessively and will be difficult to grind down properly.