EC135 does not have a particle separator. It has a sand filter system. We have a sand filter in our storage, but we really didn't use it for the last 550hrs. Depends on your enviroment. I saw saudi arabia using it, german army had it on, but removed it.
If you are intending to use your 135 in a cold enviroment, consider PW as it has fuel/oil heat exchanger.
Spot on on the dirty tail boom. PW's are really smoking. We have a metalic blue on the tailboom and you can hardly see any difference up until 50-70hrs unless you really look up close.