For belt spindles, the one skyfire uses is the best value, at about $600-$900usd depending on bearing rpm.
Microlab has a bt30 and iso25 that run in the $1000usd range.
Those are all ATC of course, which makes the most sense - even if you dont implement an atc. easier to use, easier to find economical tooling, etc.
Jianken ISO20 motor spindles are $1200usd, ISO25 $1400usd, but are high rpm only (12-30k), so not particularly ideal for a mill unless you only cut small aluminium parts or plastic.
If i was making a "mill" i would try to get a tapping centre type spindle, which uses Z travel to release the tool. this reduces much complexity in control as there is no air cylinder etc. Unfortunately because they target high speed tapping machines most of these spindle seem to be $1500-$2000 each, bt30. would be worth contacting some of the makers (mirolab, ken (not jianken), cells, royal) to see if they can make something a little simpler or smaller.
with that type of spindle, you can use a servo direct drive and do tapping, or use basically any other type of motor, direct drive. A $500 2.2kw teknomotor solid shaft motor for example running 12000rpm, a $400 delta B3 servo 750w ac servo at 6000rpm,... tons of options.