My mill has a .75HP motor and MT3 spindle. I think MT3 gives you a range of up to 3/4" tooling (which is fine for a .75HP motor) and with R8 you can go up to 1" tooling which again, I think may be too much for my machine. As a 'Do over' I'd look for a 1HP x R8 machine.
my 2 pesos, and yes you will lose about 1" in table to spindle clearence with an adaptor.
cheers,
Chazz