When I removed my stock Merc 350 260hp it had close to 850 hours and was still running perfect and not burning any noticeable amount of oil. I never did anything more than change the oil and regular maintenance. A stock 350 Mag is not stressing a Bravo drive at all. He should only have to change the gear oil and replace the seals if they ever start leaking.
Stock Merc parts are built to last and do the job with only regular maintenance.