How big a deal is replacing lower unit seals on a 1973 85 Johnson?  I am getting water in my gearcase.  I changed the impeller once & thought it was easy, but I did not change the seals.  Not really asking for directions, just an idea if this is a minor project or do I bring in a professional?

Probably a lot more than you want to get into, unless you're a pretty darn good wrench turner.   I would suggest you get some type of manual and read what's involved.  That's why most shops will charge $250 - $350 labor.  You are also going to need to make a puller to get the bearing housing out.  If you find anything wrong, and have to replace parts you will need a special guage to reset the pinion height.