Let me start by saying I'm NOT an expert- but I own a couple of different compressors.
I would get the largest compressor you can afford and that fits in the space you have available. It sort of depends on your anticipated use for the compressor, but you'll never find yourself saying "darn it, I wish this compressor ran out of air more often!!".... but you will find yourself VERY frustrated if you get one that is too small and you have to wait for it to cycle on and build up pressure time and time again.
Another thing you need to realize is, air compressors are nice to have, but they are NOT the magic cure to all of your tool inadequacies.... What I mean by that is, many of the high torque nuts + bolts that you find on your cars (rear axle nuts, front crank nuts, etc) will come loose just as easy with a good strong breaker bar and cheater pipe as they will with a 1/2" air wrench. I have run into situations where my 1/2" air wrench just could not get something loose, but it came right off when I used my 1/2" breaker bar with 4 ft pipe on it!! On the other hand, it's nice to be able to spin things loose with an air wrench.
I would recommend a good sized belt driven oil type compressor vs a direct drive oil-less compressor. If you can, get a 2 stage compressor for maximum air capacity and higher operating pressure than the single stage. Basically, a 2 stage compressor uses 2 cylinders in series, the first one compresses outside air to a certain point, then passes it to the 2nd cylinder, where it gets compressed even more- thus the higher pressures with this type of compressor. Single stage compressors use 1 cylinder (or sometimes 2 cylinders in parallel- they do NOT feed from one to the other), and compress the outside air only 1 time. Oil-less compressors use teflon lined or similar cylinder walls VS standard compressors which are much like an old car engine- they use oil to lube the "bottom end" of the compressor pump.
Now, for tools, here is a brief list off the top of my head of must-haves:
1- 1/2" air wrench
1- 3/8" air ratchet
1- tire air valve (for filling up tires)
1- blow gun
plenty of good quality quick-release fittings for each tool
Assorted air impact sockets (the black ones)- you can use your chrome sockets, but they are supposedly more brittle and prone to shattering when used with air tools. I bought several sets of 1/2" and 3/8" sockets very cheaply from Harbor Freight on sale- though a couple of them have broken (split sides) under very light use- I also have a couple of Craftsman impact socket sets and have not broken any of them yet.
I also use a small air cutoff wheel quite alot- the one I have is a cheapie but works well- it takes TONS of air though..
This should be a good start for you- do a google search for air compressors or compressor comparison to see what other info is out there.
Bret