Opti,
it's great that you're excited about diving, and thinking ahead. Gear is something many of us enjoy getting (Gear Geeks - self included), and enjoy jabbering about on here. There are lots of methods to dive safely. Knowing how to dive tables and written plans is important for fundamentals, but many of us recognize the value and improved accuracy of computers.
One question I would ask yourself is this... can I rent a computer when I'm doing my courses? If you really want to go through Adv Nitrox and Deco, almost every Tec dive shop will have a computer appropriate for your course that you can rent or borrow. Before you purchase a computer that does anything more than simply track your nitrogen and oxygen absorption, dive time, NDL, and depth, I'd really get some more experience with other people's computers first. Try before you buy is a good motto here.
As for getting a used BP/W?... the backplate is Steel: if it fits you, is in good condition, and is at a good price... just get it. Hard to go wrong. You'll learn various ways to harness it, whether hogarthian, or a more modern Transplate/backpack style. The Wing is a bit trickier. It's important to have it inspected by a shop. It needs to be pressure tested, and inspected for salt crystals, as well as cleaned. Typically this is not a problem... ask about the users number of dives, and their post dive habits. Also find out where it has been stored.. sunlight or no?
A Rec BCD and BP/W are not limiting factors until you decide to dive with doubles. Even then, some Rec BCD's can handle it. If you have a desire to go into Tec Diving, the BP/W is the right place to start. Before investing in a fancy computer, you might consider getting yourself a good lift bag/safety sausage, a proper reel (Manta Jr is great), and a good light. Before rushing into Adv Nitrox and Deco, it's a good idea to get experience with several environments... quarry/lake, ocean, bad vis, tropical, cool water, cold water, darkness (night or cave) current/drift. You might focus a bit more on the experience and the journey rather than on the goodies and c-cards, at least for a little while.
Regards,
D