Welcome to BLF, e10sno! I like the Manafont drop-in, but it is ordered H-M-L and has PWM on lower modes, which I'm not crazy about. But mine at least is very bright on High.
If you want to build your own, I think a 16mm board is the biggest that will fit in a P60. The 8x7135 2.8A driver is a good one if you want modes:
For hosts, it is hard to beat Solarforce, though you can sometimes find cheaper alternatives. For 18650 batteries, most people like the Trustfire flame 2400mAh batteries from DealExtreme:
Another thing you could do is go to DealExtreme and choose the P60 XM-L flashlight you want for around $22 and then swap the KD drivers into them.
If you don't mind spending a little more you could go all Solarforce: host and drop-in, though I don't think the Solarforce drop-ins are quite as bright as the KD and Manafont ones.