I'm actually in the same boat as you getting a watch for my wife's 30th (thankfully it's not till September so I have some time). Here's my rambling 2c:
You mention that the Datejust is out of your price range but, depending on what vintage and options (diamond dial, papers and box, etc) you require they do actually dip down to $2000 and below sometimes. Like this: http://www.network54.com/Forum/207673/message/1330719952/FS-+Rolex+Datejust+16030+with+certificate+paper+and+booklets
I tend to like the rolexes and the Omegas due to their reliability and 'lineage' but that isn't meant to badmouth any of the other suggestions or brands out there.
Watches are a VERY personal thing and unless you:
a) know your wife's tastes VERY well or
b) don't mind consulting with her for suggestions/hints -
I would tred carefully in choosing, lest it eventually lives in the jewelry drawer and not on her wrist.
Another less popular watch (from a collectors point of view, which is good for you) would be the Omega speedmaster reduced. It regularly can be found for under $2k and sports the same 'features' and looks as its much more expensive cousin the 'moonwatch' which is over $3k. At 38mm in size, it's not terribly small but you did mention the 36mm Rolex so it all depends on whether your wife would like a larger or smaller watch.
Regardless of what brand you decide upon, if you're buying used: Buy the seller not the watch! Keep an eye on the following link http://www.elitedealseeker.com/index.cgi?category=watches
which is an aggregator of a number of watch forums to see what's currently on offer. The best deals are normally private to private instead of places like Chrono24 but you do take on more of a risk in dealing direct.
Once you finally decide on the watch, let us know and pop up a wrist-shot too Edited by vaulter1 - 3/3/12 at 10:07pm