While you will be surprised at how mych better the garage looks with a two part epoxy coat, you won't like untreated cracks. If anything, the epoxy will call more attention to the cracks. You should at least consider filling and caulking the cracks.
Key is removing any wet oil stains that are on the surface. Cleaning it with a muratic acid wash after getting rid of the oil won't hurt and will ensure a good adhesion.
You need to read the manufacturers instructions and follow them. As I recall, the minimum application temp is around 60 degrees, but once its down, it may not be as critical. Your garage most likely won't get as cold as outside ambient.
The biggest problem with curing is hot tires sitting in one spot, not general traffic. Putting small 3/4" plywood squares under the tires sounds reasonable after 5 days.
I've done this in several garages. 10 years later after an initial 2 coats and it still looks excellent.