BCD's are pretty simple, so if you buy a new BCD, get one with generic parts (inflator mechanisms/dump valves). For example, my Diverite Transpac and Oxycheq wings use such generic parts. Also try to get a BCD that can use modular/generic weight systems, if you don't use a weight belt.
Regs are more complex, often not generic parts but require OEM 'special' parts. Soft parts are often generic (O-rings/etc.) but an Atomic piston ain't gonna fit a Scubapro reg, for example. The good news on regs though is, if all else fails, you can go on E-Bay, Scubaboard classifieds, etc., and select a cheap, low-mileage and quite usable reg whenever you want, the brand/model doesn't really matter too much, as long as it's a common model produced by a mainstream scuba brand. There are plenty of people who consider regs to be disposable, they'll dive it a few years, then E-Bay or trash it rather than spend $ 150 on a full service, so they couldn't care less about long-term product support.
Being the gearhead that I am, this year I have rounded up and had serviced a number of Scubapro MK 5's / 109's, just because I like the vintage look/clean design of the classics, but SP has finally completely phased out OEM support (at least for the MK 5) so I have gone to considerable effort to get help bring those units back to life using 3rd-party parts/service. That being said, the vast majority of divers are not gearheads and couldn't care less about regs other than as a 'tool' for breathing underwater, so for them, spending $ 150 every 5 years on a cheap but functional E-Bay reg meets their requirements.