Experience with Sherline CNC Mill

It's not hard to fit either mill with a 3rd-party spindle
assembly, but I'd be hesitant to recommend the "Wolfgang", which
is basically a tiny RC airplane motor attached to a collet
assembly through a couple of bearings. Comparing it to a NSK
spindle is like comparing a go-cart to a Ferrari, IMHO. Here's a
continuing discussion about it on CNCZone (a good forum for
matters CNC) 

It’s true you get what you pay for. The Wolfgang Engineering spindle
is something like a tenth the cost of the NSK system. There are
other options too, like air powered spindles, which have insanely
high RPM but are noisy. There are some other low cost options too. In
my case I do wax mostly, and have found the 10,000 RPM of the Taig
ER-16 spindle to be quite adequate