I would suggest that if you are able to change the prop shaft seals you are capable of replacing the others. I'm very new to outboards but since you mention your friend's mate's Yammy, and also that you have a Mariner, I am assuming that yours is a Yamaha built Mariner (like what I have). I am in the process of replacing the water pump gaskets and the seals in the top of the lower leg assembly. Mine has an o-ring and an oil seal for the cover that the lower shift rod protrudes through (very easy to change both), three gaskets in the water pump assembly (not to seal the lower leg unit just the water pump assembly), as well as an o-ring and two driveshaft seals for where the driveshaft exits the lower leg assembly and enters the water pump assembly. The seals (2) in the bearing carrier (prop shaft seal) looks to me to be the hardest to replace but I could be wrong. The Yamaha dealer I got my parts through said it can be tricky getting the prop seals out without damaging the alloy bearing carrier - he might just be looking for work but he struck me as genuine - and if they aren't leaking he wouldn't mess with them. I know they aren't leaking now as the engine has been sitting unused for years and a lot of oil came out when I drained it (not milky either so is promising).
I wouldn't be concerned about the gaskets (fibre washers I think) not being a tight fit on the "plugs" (bungs, etc). They are designed to seal between the under side of the plug head and the flat machined surface of the lower leg (gear housing) under the gasket, not around the threaded area. If you replaced them making sure that no grit came out of the gear case they should be fine.
I would look at replacing the oil seal and o-ring for the plastic/nylon cover around the top end of the lower shift rod, and the o-ring on the underside of the water pump assembly for a start (easiest to sort out) and inspect the oil seals around the drive shaft. Maybe tip the lower leg upside down and see if oil comes out through there with the water pump assembly removed.
I'm no mechanic and I have had very little exposure to outboards, check what others post also. You probably should be a little more specific about what outboard you have too, serial number is best but hp and year would suffice (mine is a Mariner 55hp - 2 cyl 2 stroke, serial starts with 663.