I've been putting a mountain bike together from a 1997 Marlin frame as a learning project. At this point I have a RockShox XC30TK - Coil 80mm 26-Inch 9mm fork installed. Now I'm focusing on the wheels. I've been told that my frame will support a front wheel that is disc brake compatible, so I went ahead and purchased an Eastern Nighttrain wheel. I now have several questions that I hope you folks will help me with.

1) Would a Mavic 2011 Crossride UB rim brake mountain wheel be okay/compatible for my rear wheel? I've been told the only way that both wheels could have disc brakes is if I installed an adapter on the rear--something I'd rather not deal with.

2) What size disc rotor should I get for my front wheel. I don't really understand what the different sizes mean. If it's relevant I'll primarily be riding this bike on the street, maybe some mild trails occasionally. I'd like to keep the weight as low as possible without compromising safety.

3) Why is there such a range in the price of disc brake rotors? I've seen anywhere from $20 to $150. What may be gained by purchasing a higher end one?

Thank you for your time.