I would look at code requirements for wall assemblies. I think R19 is common for places like you describe, but I've never lived there so I have no real clue.
The big criteria is preventing condensation, which is what minimums are based on assuming a "typical" residential heat source. Since it's a garage that's not likely to be cooled and you're not trying to stay at 75F all day, so code minimums should be OK. But when it comes to insulation, more is better.
Doing partial areas, like roof/ceiling will help more in the summer than in winter (but can help prevent drips from condensation). But it really takes doing the whole space to make it right. Including doors.
