1. Get something with the same manual of arms as your primary, unless you plan on practicing extensively with both.
2. Plan on practicing extensively with both, anyway.
This still does not make #1 a bad idea. The "baby" version of your primary is a really good idea.
3. If you cannot afford it, wait a few paychecks. Do it right the first time.
4. Snub-nosed .38s/.357s are about the ideal BUG. They're sized right, they go bang every time, the manual of arms is dirt simple (the original point and click interface) and essentially the same as any non C&L primary, and they're powerful.
5. Ammo/magazine commonality is a nice plus, but hardly a dealbreaker. I carry a 4506 and use a 642 as a BUG.
6. Invest in a quality holster. Explaining to your supervisor how your BUG went AWOL on a footchase would not be fun. The Galco Ankle Glove is what I use, and it is rock solid.
7. Make sure your PD will approve whatever it is you decide to carry.
Assuming I had $300 on hand and I was allowed to carry a snubby .38 or .357 as a BUG, I would shop around for a lightly used J-frame, get it, and save up a few more bones for the Galco holster (assuming you want it on your ankle), and then a little bit more for an IWB one for OD.
My very close second choice would be the baby glock.
Do not skimp on the leather.
Mike