You should design using the measurement system which makes sense to you and which is suited to your projects — I switch back and forth, using Imperial measure for anything which will involve thirds or sixths (breaking out a PostScript Pica rule if need be). I’ve done a number of projects in metric and just used Imperial tooling for CAM and just done those measurements in metric (mostly I use metric for CAM anyway), so no need to switch even if you use Imperial tooling.
You will be spending more for Endmills than collets in the long run, so it makes the most sense to work out what sorts/sizes of Endmills are most readily available to you, then order collets to match.
If that is metric, then the Elaire Corp. collets are excellent quality (I have the full set supplemented by the Carbide 3D 1/4" and 1/8" precision collet set) and I can recommend them w/o reservation based on my having the full set and them all working well.
One thing which you may wish to do is to try to hold out on this until the ER-11 collet version of the Carbide Compact Router is released — that should make sourcing collets much simpler. Check in w/ sales@carbide3d.com for availability.
One conversion option is to get a 5/16" (8mm) collet and an ER-16 adapter — you have to limit your RPMs, and it increases stickout quite a bit, but folks have been doing it:
for links and a cautionary note: