If you parents will let you, get the 450R.
Im not going to play daddy and tell you to get somthing slower. Thats what your parents are for. Im sure you will be ok with the 450R or 400EX with proper supervision. If your dad used to race im sure he knows whats going on. Besides, did you guys think maybe dad wants somthing he can ride too??? If your dad is real smart he could adjust the throttle and detune it until you are a little more expirenced.
I was riding a KX125 when I was 14.
2000 400EX
416, Wiesco 11:1, FDO Studs, XR head gasket, Port and Polished head, 3 angle valve job, Hotcams Stage II, CRF 450 Timing Chain, Jegs Oil Cooler, K&N, EHS Air Box Lid, Choke removed, Sparks X6, 42pilot, 168main, Gusseted Frame, Ohlins, OMF Scoops, Shaved fenders, DG Pro-Pegs, AC front bumper, Armadillo .250 skid, 909 bars, Twist throttle, Maier Handguards, ITP beadlocks, Kenda Klaw XC's, LSR anti-fade hub, Mezaros Billit Bearing Carrier.