Ordinary "low odour" thinner meant for oil based house paints works really well for the sticky goo and I've never seen it attack any sort of automotive paint. Hell, it doesn't even attack the house paint after it's dried. I'm not 100% certain but I think the same stuff is sold under the Varsol cleaning solvent name.
And removing the stickers this way only works if they aren't clear coated over. Otherwise you would have to score the clear coat all around the edge first.
The prep work for painting is pretty easy. You just need to use 1500 or 2000 grit wet or dry to take off the shine so the new paint has some tooth to grip. But keeping the freshly sanded surfaces clean of any finger oils or other contaminants is a big problem so check with the paint shop first. They might convince you that it's easier or better to let them do it. They are the experts. I'm just going from what I've heard from some painter friends.
