Hi,
firstly the NEMA42 or NEMA52 is easy. The number refers the the size of the servo across the flats. Thus a Nema42 is 4.2 x 25.4=106.7mm across the flats while a NEMA52 is 130mm across the flats.
NEMA is a US designation while many of the devices you'll be looking at will be Chinese made simply because they are so much cheaper, and by-in-large the Chinese is all metric....and so you'll not find NEMA42 size.
I use and recommend Delta servos, a Taiwanese brand made in China. They are good quality, support, documentation and most importantly free set-up and tuning software at fair prices.
There are cheaper Chinese brands and they all seem to work OK but the support is questionable, the documentation poor or worse and often NO set-up and tuning software. If you've never had a servo before
don't go there. Sure it'll be cheap, but you'll want to throw it in the junk pile before you get it to go.
I use 750W Delta B2 series servos on my machine. The are 100mm across the flats. You might say they are the 'metric equivalent' of a NEMA38 (actually NEMA 38 is 96mm across the flats).
They are 2.4Nm S1 rated, 7.1Nm overload, 3000rpm rated and 5000rpm max. I pay $438USD plus shipping for them, and I've bought 7 in the last three years.
https://www.fasttobuy.com/flange-80m...er_p28084.html
This one, from the same company is 2kW S1 rated or 6.4Nm, 19.1Nm overload at 3000 rpm but with a max speed of 5000rpm. The kit (servo drive and cables ) is $895USD plus shipping and would make a kick-arse spindle motor.
https://www.fasttobuy.com/delta-2kw-...nc_p33936.html
Craig