I think the best thing I ever invested in was the Makita LION LXT set: in particular the impact gun, angle grinder, and drill are all heavily used. I no longer have to wait for power.
Other little tools are obvious: compression tester for compression and vacuum testing, you need the etorx to drop the subframe or the bigger Torx to pull things like the tensioner but I only learned the value of some of them after people showed me how much easier they make life for you.
Examples worth mentioning:
- Good headlamp: discussed here. best work light for cramped areas?
- Detachable zipties: this might be more useful in a racecar than a show car but did you know they make zipties you can detach without any tools? I didn't until a buddy who works at 3M showed up with a few packets for me for my first race. I now keep them in stock and buy more when. Example
60 Pcs 12" inch Heavy Duty Releasable 50 Lbs Nylon Cable Zip Tie Black Wire | eBay
- 9mil nitrile gloves: another item I keep regularly stocked up on from HF. Perhaps I'm a pansy here but I don't like getting covered in grime. I use them until they tear and get another pair. This isn't a condom here so I don't see the value in thin or powered ver.
- Coveralls: I often don't have much time to work on the car (30 min here, 1 hr there) so being able to go from under the car to presentable in a short amount of time is useful. I've torn the knees out of a bunch of them (to the point where even using old jeans as patches wasn't sufficient) but the last pair I've bought seems to have lasted far longer than the previous sets. I think I looked for the thickest weight possible since I never found full coveralls with double knees. It also helps that it hardly ever gets hot enough to sweat in my garage in SF. https://www.amazon.com/gp/product/B005SW3Q7E
- 40 or 60pc Tap and Die set from HF: working on an older car means dealing with crudded up bolts/holes. This $40 set has been invaluable for the # of bolts/holes I've been able to clean up before reassembly. Since I bought this set 3-4 years ago, the only addition I had to make was a set of NPT taps/dies (<$8 from HF) so I could install a plug in my GT40/Explorer lower intakes EGR piping.
- Offset Hose removal tool: makes removing rubber hoses without cutting them even if they've been attached to a metal pipe for a long time a TON easier.
- Shop vac: don't just get the cheapest 5 gal one from HF; spend a few more bucks and get the one with a higher HP rating and it will still give an acceptable amount of suckage for a longer period before you have to clean the filters. I didn't learn how awesome this was until after I bought one and was given the opportunity to upgrade to a friend's unit (sold mine on Craiglist for a few bucks).
- Crows Feet sockets: Sometimes you get in a random spot where you can't quite tighten a given bolt (key example: bolt ut that holds the distributor down is hard to reach with a socket or even a wrench on a 5.0).
- 6pt sockets (deep and shallow) in addition to 12pt sockets: after completing a set and buying all the missing sizes in a given set of 12pt sockets, I found the next good investment of money is buying a 6pt set. Why? No more rounding bolts. I reach for the 6pt set whenever it's convenient and for the tbird, I found I mostly use the 3/8" tools vs 1/2" and 1/4" drive.
I don't have the room for a monster compressor and considering my garage is also my basement (living on a hill), running a compressor would be too disruptive for my toddler sleeping (when it was just my wife I didn't worry about it so much).