Would like to custom-build a bike but I'm not sure on what parts to buy
Upgrading parts is likely to be the most expensive way to get the bike you want. Depending where you are, second hand is the way to get a good bike for the lowest $$$, provided you are careful. The next least expensive option is to go for an end of season new bike. In the end, it costs a lot less if you use the proceeds from selling your old bike rather than upgrading it. 

29'ers  (I have one), give quite a different ride to a 26. Try one out to see if you like it, however if you stay hardtail, I think it's a good way to go. If you are a heavy 6'6, consider the 29'r has longer spokes and the wheels are therefore weaker than a 26 (all else the same). As I am under 70kg (150lb), its not something I have to worry about. Also think about the shock - is it stiff enough for your weight, although the only thing that makes a significant difference at this level of bike is more money, and unless you are over 250lb you should be OK (Trek states 136kg max fro their bikes). 

The Mamba looks like a reasonable fit for you in terms of budget and the riding you propose - it will hold up with OK quality parts, I imagine a bit heavier than ideal, but it won't be a dog either. Trek make good bikes and have a good reputation (where I live) for support - which is important.