Yes the rear diff is better when locked for drifting as you stated, the wheels spin equally. You can do that by taking out your diff, take out the ball bearings, and put the diff back together without the balls in it, tighten it up pretty tight, and make sure you cant spin the diff by hand. It would then be locked, so it cant spin seperately.

Yes the front one way is good to have, it gives you sort of an "e-Brake" effect. It's used to help cornering and such, when drifting it makes it better to do transitions, like drifting to one side then quickly drifting to the opposite side. It isnt a MUST, but most people prefer it, if you have a technical track/course with fast turning then it is alot better. I have one also.

You can also lock the front and the rear instead of having front oneway front, with locked rear. The second option is what most people use, and in general better. I have seen and read about some people using both locked, but i would go with the front one way. After that, you can pretty much just slap on drift tyres and get drifting xD.