Cant offer advice on what brand to use for the mirror, but I would go OEM. As far as what things you should do right away, that all depends on how many miles are on it. Use this site http://www.wkjeeps.com/wk_maintenance.htm
to figure out what maintenance you should do based on how many miles are on the Overland. When I buy a used car I assume the previous owner hasn't done any of the scheduled maintenance. Might be overkill, but I like playing it safe. I'd also do a check on the brakes/suspension. Most of the maintenance stuff can be done by weekend mechanic with the right tools and the How-To section on the site covers just about everything.
As far as the mirrors go, might wanna try your luck and go to www.car-part.com
and see if you can find your mirror(s) in a junk yard in your area. Might be the cheapest most effective option.