From the figures I've found, a similar mill using an older posidata drive used 3/4hp motors combined with a 1.1hp drive.
The motors that are currently on the mill, come in around 1kw from what figures I've found (they're SEM) but the owner wants them as spares for the bridgeport interacts he's got.

I know steppers will mean a hit on performance, but it's how big a hit I'm trying to quantify.
The DMM 750W servos should come in under £500 each before shipping (depending on how customs class them), comparable in price to the HBS86 closed loop steppers. Performance wise I doubt there will be that much practical difference. Steppers will produce less torque at speed, but this mill won't be cutting at speed, so low speed torque is more important.

I really want something closed loop, and all the options start around £500.
Options I've priced are Dynomotion K-flop + Kanalog, combined with Nema 34 steppers and US digitial encoders, and high voltage drivers which is around £1700 all in.
HBS86 closed loop are around 500 each plus power supply.

Thanks for the suggestions guys.