Wow Iggy, Sorry I missed this first time around. SNR was to high I guess.
I've done a lot of what you want to do. I've even used Bosch Colt routers
as mill spindles. Not just for engraving, but for aluminum milling as well.
I also made a mount for a spindle mounted to the side of the quill on my
knee mill.

I'd note that I have high speed engraved steel, aluminum, and brass. It can
come out decent dry, but it looks so much better if milled with flood. I
run SC520 and distilled water at 6-8%.

The Bosch Colt will work wonders. I've clocked a few of them from 32-34K at
top speed. I've worn out a bunch of them, and I still have one mounted a
spindle on my small CNC wood router. It also has its issues. The biggest
one is its a wide open universal brush motor. You might be able to run
flood for engraving as it doesn't throw that much water around with a tiny
little engraver or ball mill, but you need to be aware there is a shock
risk. If you are tempted to do a little more with it you can. I have, but
you will pack them full of chips eventually. I did. If you decided to go
with a Bosch Colt Precise Bits in Colorado makes precision tapered collets
with their own version of an eccentric ring for that router. Its been a
while since I rebuilt one, but I seem to recall its a plastic spindle nose
as well. If you do really long jobs you can melt it. I have. LOL. I've
burned up and wore out a few of them. Porter Cable 690 routers too.

Today I have three mills with 24K water cooled spindles. I use one of them
for all my engraving. They do cost a lot more a spindle, but if you all you
are doing is engraving any of them will work fine. Your horsepower
requirements are miniscule. Almost uncalculateable. A 3 bearing 24K .8kw
is overkill and they aren't all that expensive. If you really want to push
it they make spindles like that for upto 60K. 24K is plenty for me. I
still sometimes do some milling with that machine too.

Somewhere around I have a bracket mount a 65mm .8kw spindle onto 3.375 quill
on my Hurco mill. I used it for doing fine detailed 3D milling as well at
one time. If I think of it tomorrow I'll see if I can find it and snap a
picture.

P.S. I'm not a big fan of Bob Warfield, but he has put together a nice
collection of other people's ideas on his website.