I've finally bought the ER16 collet chuck and set of collets for my big brushless motor. I thought that it'd compliment a kress spindle nicely when I can afford one of those since this motor will do up to 6000rpm, and the kress from 5000 to 25000rpm.

i cant see you having any worries with that monster!! are you going to put more bearings at the nose?
i used my 700w motor to do my test cuts(0.6mm depth) the bind with using that at low revs is the amount at which the revs drop under load, it needs factoring in.
i really need to re-work my 2000w spindle housing to fit the 43mm spindle mount (i bought a boat load of hss 5mm 2 flute of ebay to try in it, nice and cheap)
im not sure the single flute cutters are that important if you can get your revs down, youll have no worries there
my take was that with plastic the bigger the chip the less chance of melting but i am doing a lot of guesstimaiting at the moment, ill give a few diffrent cutters a try and see how i get on (delrin is pretty nice to machine)