If you are trying to get an MVP, look for one that has been trained well this season. You might not find many of these with your current cash, so then you might want to wait until the draft, since you are in D5. There are 5 expansion teams and 2 other teams that haven't logged in since at least 4/21. That technically moves you 7 spots up and gives you a better chance for a good draftee.
To answer your second question, if you have a BIG with MVP potential, they are probably not making the NT. If it is a guard, then you have a good chance, if trained accordingly. Also, if you have an SF with superstar potential, and that SF is a balanced one, then you might have a legit shot for the NT. For example, look at Bill Weeks
(11772141) and Irwin Velasco
(8868669) . There is a PF that has superstar potential as well (Frank Tanner)
(14602057) , but this doesn't happen too often.
I would say, first wait for you league draft. If you buy an MVP player right now, they will turn 19, unlike if you drafted someone with superstar potential that was 18.