You want to buy bitcoin and be anonymous ? That's the very least of your problems !!
So you go to Tor and take great precausions to keep your name secret .
Well ,being that most people buy BTC with the hope it goes up in value , and that is where your real trouble lays .
How are you going turn your gain back into dollars ,and be anonymous ?? 
You can't , and then comes taxes ,yep, your fucked no matter what you do.
Buy BTC with after tax dollars ,and have no fear of being "caught for something illegal" then sell and report your gain, just like the rest of us honest patriotic people do.
Only a low life would be scheeming how to cheat the government .