Re: What to look for?
Parts and service are still available for your 97 Force, so don't worry too much about that. Check compression on all cylinders. It should be 90 psi or more and within 10% on all cylinders. Check the gearcase oil for water intrusion by draining a little from the drain hole in the lower unit.
You should get an on-the-water test drive with any boat that costs more than about $1000. The outboard should start easily, and run smoothly at all speeds. It should shift crisply into forward and reverse.
If it has tilt and trim, those should work properly as well, without any creep or bleed-down.
There should be good cooling water flow, but you should also have the impeller replaced unless the owner has a receipt showing that it was done within the last two years.
Parts and service are available at Mercury dealers.