Paddy:
I'd have to say the first piece of "machinery" I ever bought was a compressor. Don't know that it's actually a "machine tool", so perhaps we're straying off topic here, but I'd still put it at the top of the list. As others noted, you're forever blowing things off with it, and filling tires. It also opens up lots of other options that might otherwise not be a possibility.
In the "small tools" arena, the air-wratchet and impact wrench are lifesavers, as I couldn't have removed old suspension bushings with such a simple contraption without it (
elan-f15/removing-year-old-front-suspension-bushings-with-photos-t23684.html).
For any grinding work, a pneumatic disc grinder has also been really handy, while a die grinder has allowed me to do surprisingly precise work in relatively small spaces. While you could probably do much the same with electric powered tools, the pneumatics are lighter and more nimble, in my experience. Finally, you've got options for other solutions in the future. I (probably impulsively) bought a blast cabinet that will make life much easier for me as I strip down and refinish suspension and other bits on the car. Hard to know if I'll use it after this rebuild, but the point is that I have the option because of the air supply I already own. I also plan to make my own soda blaster (
http://www.aircooledtech.com/tools-on-t ... a_blaster/) for cleaning my Weber carb bodies, and again, couldn't do that without the air.
Regarding the compressor itself, I opted for something that I could use to spray a car if I wanted to (done a couple in the past, so maybe I'll be just that crazy again someday). I didn't want it running constantly, and was glad for that choice when I spent a day grinding on the frame welds a few weeks back. It's loud when it's on, but it's not on for more than a couple minutes, and then can run many tools for 10 minutes straight before needing to recharge.
I opted for an upright (major space saving design) 80gal, single stage model from Campbell Hausfeld. It delivers 135psi, and about 16CFM @ 90psi. In short, plenty of air for anything I need. It was something like $900 several years ago, and looks something like this:
I actually started this thread looking for advice on machine tools, specifically a combo lathe/mill vs. separate units. Here I am, taking the "buy big" approach, which may not be right for you. If you're mostly about pumping tires, dusting things off, and perhaps running an air wratchet, you can probably go much lighter, smaller and cheaper, which is good. The downside is that you'll probably run into a situation where your compressor won't be able to do something you want, and it'll generally run more frequently and be much louder. I say this assuming you just want info to help make a decision. We all know when our compressor is running, but some of them have a higher pitched, dental drill hammering, while the bigger compressors sound more like a motor running, at a lower pitch. This last item is something I didn't think about, but was brought home when working with a friends small compressor for the day. Bit of a headache by the end. Made me love my decision that much more.
I'm rambling here, so my last comment is that I just bought a mill/lathe combo. I can easily see regretting it when it's sat for years and I only used it twice
. I've never felt that way about the compressor, even during the years when I only ran it every 6 months to blow dust off things. Every Lotus owner should have one, and big is good, but expensive, but only done once. Oh, and the upright takes barely more floor space than any most other models, actually.
Hope that helps,
Dave
P.S. I've got to start being more brief in my responses, don't I.