I need a 240v outlet right now, and I want to do a full rewire of the
garage in the near future. A subpanel sounds like a really neat
idea. The main panel is just about full already and I just like the
idea of a separate panel in the garage.

But,

A. It looks like it's going to cost a lot.

B. I really don't know what I'm doing.

If it's the right way to go, I'll learn how to do it right and spend
the cash.

Alternative: I've seen those 2-circuit-1-slot breakers. If I take a
few of the existing circuits on the main panel and switch them over to
those, it'll open up enough space for a 240v breaker for the MIG
welder and a few 20amp circuits so my air compressor will quit making
the lights go out. Wiring that up is within my existing knowledge.

Is that a half assed solution? Sometimes the simple route is the
best, but it's my house and I want it done right.