BP&W is a modular concept. It can grow and provide for your diving needs as they evolve.
Start with the harness. Get a basic 'Hog' harness. Once it is adjusted correctly it is like a second skin. Only upgrade to a complex (and expensive) cinch/comfort or deluxe harness if you later feel it is necessary (most don't).
Then decide on your backplate. The choice of stainless steel, aluminium or plastic will be determined primarily by your buoyancy requirements. However, weight plays an issue if you will be travelling regularly with the kit (and subject to excess baggage charges). Give us an idea about your diving (current weight used, wet/drysuit buoyancy etc) and we can help you decide what is best.
Then decide on the wing lift that you need. Search for the 'Wing Buoyancy Calculator' thread here on SB. Typically, you will have the option of 18lb, 30lb and 40lb wing sizes for a single tank wing.
Choice between brands is dictated by your budget. Wings vary in quality and the material used for construction. Brands differ in respect of customer service and warranty. Don't think that you have to buy a 'complete system' from any single manufacturer - in most cases the backplate, harness and wing itself is inter-changable between brands.
When you move into tech diving, you will only have to swap the wing. You shouldn't compromise on a wing that may be advertised as 'good for both single and double cylinders'. There isn't a wing made that is optimal for both. You only have to undo 2 screws to change a wing over... so it isn't a drama to swap between single and double cylinders when you want to.
The Oxycheq Mach V Extreme is the most rugged wing on the market. Whilst expensive, it offers benefits for wreck diving, where abrasion and puncture are hazards.
Halcyon is expensive, but they have a consistant high quality and the warranty/customer service is excellent.
DSS is represented here on SB by the owner, Tobin (coolhardware52). He does an excellent job in providing personal service and advice to his customers. Highly recommended.
OMS and Diverite make good kit, but (IMHO) are more middle-of-the-road in respect of quality, attention to detail and customer service. There's nothing wrong with their kit. Consider these as 'budget' options.
I have the following:
Halcyon Ali backplate and hog harness (for travelling).
Custom Divers SS backplate and hog harness (for home).
Halcyon Evolve 40 (for doubles)
Custom Divers TDB Double Bladder (for doubles, but I rarely use it now)
Halcyon Eclipse (for singles).... will be getting an Oxycheq Mach V Extreme 18lb (for tropics) next.
I mix and match the kit depending on my requirements.