When my wife and I bought some property a couple of years back, the first thing I did was build a 40 x 60 garage building...it was completed over 18 months before we built our house.
That was a wise move on my part because I undoubtedly would have come up with ways to spend the "garage money" on other matters had I not built it first.
The most useful addition to the garage has been the two-post lift I installed. It was money well spent and has made some very unpleasant tasks such as transmission installations, undercar line running, etc. much less painful (and time-consuming). Also very handy are electrical outlets every ten feet, and "strip outlets" above the workbench area. I prewired for 220v in three places, which made hooking up my lift a breeze.
I do not have a chain hoist but wish I did. I also do not have drop-downs for air lines but think that is an excellent idea. Also, you cannot have too much lighting; there's nothing worse than working in a dim garage.
I have a CD player that I bought at an electronics store for less than $100. I hooked up some leftover speakers from a now-deceased surround sound system and now have a nice stereo to listen to. After the lift that is actually my most-used and -appreciated addition. I also have a TV and VCR but don't use those much.
A small refrigerator comes in very handy, especially for sitting around and staring at the cars after a day of working on them. The WC idea is good, too. I have all of the pipes run for a toilet and sink, but haven't hooked them up yet; at the least you can do that for future additions.
A parts cleaner comes in handy. A good idea I got from somewhere was to have two work benches, one a "dirty" one and one a "clean" one. The "dirty" has a grinding wheel and drill press, next to the parts cleaner; the "clean" one is off by itself. Disassemble stuff, grind/drill on stuff, clean stuff in the cleaner at the "dirty" bench, then reassemble it all, etc. at the clean bench where you won't have metal shavings, etc. getting into things.
You cannot have too much shelving. Also, a storage closet can be invaluable in storing oversized parts, greasy transmissions, etc. or other stuff that would be an eyesore out in the open.
Some sort of floor treatment would be desirable. I haven't done it yet--2400 sq. ft. is pricey!--but I am looking at flooring from Racedeck (
www.tuffdeck.com). I have heard good things about the stuff and like that it is not a coating, but tiles that can be replaced (or removed and sold) if necessary...
Have fun! Get yourself a Griot's Garage catalog and start drooling--