First off, if the budget is painful, the answer is used gear.
The problem with used gear is that you often can't go and look at it and try it on, so you have to know what you are buying. Sometimes the gear is for sale because the person just didn't get into diving . . . and sometimes the person didn't get into diving, because the gear they bought was awful.
A BC has only a few basic functions to accomplish. It has to retain your tank, and preferably in a stable location in the middle of your back. It has to provide flotation at the surface, and compensate for the loss of lift from your exposure protection at depth. And it has to be willing to vent air on ascent. Those are the BASIC functions; to that, some manufacturers add holding your ballast or providing storage for your small accessory items.
To hold the tank stable, a BC has to fit -- and that means it has to cinch down snugly around you, so that the tank can't slop from side to side. BCs which are all fabric, and don't have any rigid attachment on which the tank can sit, tend to be sloppy. Always try a BC with the exposure protection you intend to use with it, so that you KNOW that a) it will be snug, and b) you can GET it snug when your mobility is a little compromised by neoprene or a dry suit. A mobile tank is a manageable annoyance for an experienced diver, but can be the difference between fun and a bad time for a novice.
BCs have to have enough lift for the purpose -- enough to float your gear (without you in it) at the surface, because sometimes you have to take your gear off before getting back in the boat. And enough to compensate for the loss of lift of your exposure protection at depth -- which is minimal in warm water setups, but can be significant in cold. But too much lift is counterproductive, because big air bladders can hid air from the vent orifices, and make buoyancy control on ascent difficult.
There are two basic styles of BCs, jackets and back-inflate. Jackets wrap the flotation around the diver -- this makes it very comfortable and easy to float on the surface, but makes the BC bulky on the front of the diver. Many such jackets also have a lot of padding, rendering them intrinsically positive, so you have to carry pounds of lead just to sink your BC. In warm water, this isn't a big issue, but in cold, you may be rationing every additional pound of lead.
Back-inflates are not as reassuringly stable on the surface -- you may, for example, have to lean a big back to be stable. But they leave the front of the diver unencumbered, and often have less padding so they don't require as much ballast to sink.
Kind of the ultimate evolution of the back-inflate BC is the backplate system, where a rigid plate sits on the diver's back, with a separate air bladder, or wing, that attaches to the plate. This leaves the front of the diver clean, holds the tank stable, is infinitely adjustable (because the harness is simple and can be made as long or short as you need it to be), and allows the purchase of a wing whose lift is appropriate to your needs. The downside is that you have to provide some kind of other storage for small items, and have to figure out how to manage your ballast, because backplate setups don't come with weight-integration.
If your budget is tight, I'd suggest watching SB ads and your local Craigs list, and if you find something that intrigues you, come here and ask us about it. In the meantime, you can go to your local dive shops and look at, feel and try on a variety of styles, to see what seems comfortable for you.
