I have a couple suggestions.
First, I have to echo part of blue duck's advice. Go with something good (in case you stick with it) but cheap (in case you get issued something after training or decide you want something different after trying some of your classmates' guns).
The Ruger P89 is a great gun (I owned one so I can attest to this). It is very sturdy (you could probably run over it with a tank). It is accurate. It is reliable (I had probably 2000-3000 rounds, certainly 1000-3000, through mine and I never had one FTF or FTE). Best of all it isn't overly expensive (it will probably be in the low to mid $400's, though it may be under $400 in many areas). It is aluminum alloy so it isn't too heavy but it is quite strong. The high caps hold 15 rounds.
The CZ 75 is a great gun (only know from reputation, I don't own one YET). They seem to have all the advantages of the Rugers plus better fit and finish for just a little more money (we're talking a $20-40 difference usually). The standard model is DA but its safety does not work as a decocker so you'd need the CZ 75 BD Police (decocker), CZ 75 DAO (double action only, of course), or CZ 75 D Compact (decocker). The full sized models are capable of holding 15 rounds, I think the compact is 13 (though I'm not 100% sure of that).
The Taurus PT92 is a great gun at a good price. Many people disparage Taurus products but their revolvers and STEEL FRAMED autos are great (I have owned a revolver, only know the steel framed autos on reputation and word of mouth, though I'm probably getting a PT911 by summer, hopefully by spring- I do put my money where my mouth is). This is designed after the Beretta 92 with some improved features and a much lower price. The safety is moved off the slide and put on the frame (more natural and quicker for most people) and the safety works as BOTH a safety (for cocked and locked carry for a SA first shot) AND a decocker (for your politically correct police dept). I think the high caps are 17 rounds (I could be wrong). Best part- the price is in the $400's (mid to high usually).
Also, the Taurus PT911 if it isn't too small. All the advantages of the PT92 (though it isn't a 'clone' of the Beretta) in a smaller and lighter package. No high caps available for civilians (no pre-ban mags) but I don't know about law enforcement. This gun should be found in the low $400's
...Judging by popular opinion around here, 9mm's main (only?) advantage is the higher capacity available, so should I just get a big Glock with 18-round mags, or should I look for some other attributes?
A good self-defense load in 9mm is quite effective so there is nothing wrong with it. Of course it isn't a .45. In a full-sized service pistol its main advantage is capacity as you mentioned. Also, many 9mm pistols seem to be more reliable than .45s.
The other big advantage can be in size and weight. If you look at the rounds side by side there is a big difference in the size of the 9mm v. the .45 so you can get a much smaller and lighter pistol in 9mm. For example, the Taurus PT911 is nearly a service pistol in size yet comparatively it is very small and thin and is still capable of holding 10 rounds. Many manufacturers make compact and even pocket sized 9mm pistols, some of which hold 10 full rounds.