You can get a lot of parts at the site below - you may want to take a look and see just what all they have for your 850:
For older Mercs, I recommend earplugs!https://www.oldmercs.com/Default.asp
And of course there are the Sierra aftermarket parts at places like http://www.iboats.com
Hope your experience coming over to the "Dark Side" is a good one! I'm mainly and OMC guy too, but have a few Mercurys.