Others have said it, but let me repeat... buy used gear. Especially at 15 years old, used gear is where you want to be looking. Likely as not you haven't finished growing. Buying a BC new is just throwing money away since it's possible you're going to outgrow it in a year.
I really like all the hobbies I do but most of them are fairly money intensive, at least at the start. As a result, I bought used gear for my diving. I only spent $350 and got a scubapro reg, oceanic octopus, a console including compass, spg, and oceanic computer that does nitrox, and a BC. So for less than the price of some 1st and 2nd stage sets (not including octopus) I got an entire rig. I spent another $130 (a little on the higher side---one of the downfalls of Scubapro, I hear) getting the reg serviced because it had been a couple of years.
Look here on the Scubaboard classifieds. Look on Craigslist and, as a last result, eBay. Do some research on brands you want and, if you can, specific models you're interested in and even post your own "wanted" ads here and on Craigslist. Whenever possible, put your hands on the gear before you buy it. Read up on specifics of what to look for on individual gear. Try it out underwater , if you can, before buying.
After all that, I'd personally (and with very limited experience) buy a BC or reg first. BCs can be very personal items and being familiar with yours will help you be a better diver. That said, for the most part rental BCs are going to be basically the same in most places you go. Regs are your life support and I personally think they should be the one piece of gear you really own if you only buy one. The issue of mis-matched hose connectors will be rare, though hose length will definitely vary and occasionally be a nuisance. I'd buy a computer last because you probably have a dive planner and you can use an SPG, depth gauge and a watch to perform all the necessary functions of a dive computer without all the bells and whistles.
