Sounds like a good preventative maintenance schedule Brett..
One other thing is to regularly remove the prop and check for fishing line..
As long as the seals are intact and there is no water in there I'm not sure how much is to be gained by changing the gearbox oil every 50 hrs though...
I recently looked at a 2nd hand yammie 4 stroke 25hp on a stand as he had already fitted the new motor..
Ran and sounded fine but the prop was beat up badly and it felt a bit iffy when i put it in gear so i cracked the gearbox drain plug and it was totally empty...
I passed on that one..