HP 500 EFI or carb are same basic animal, you can dig out a lot of info on here about them.
Basics, 200 hrs change the valve springs, spend some bucks and get quality springs, shims and retainers. Check the chit every season for signs of problems. Check the Crane roller rockers seasonaly and plan to replace around 200-250 hrs, or just change to the Crowers and forget them. Plan on changing the roller lifters around 250-300 hrs. Other than the valve train they are good motors, most all failures have been valve train related. When I had to rebuild mine it was a broken valve spring that caused it, dropped a valve, was very lucky did not do much damage other than mark the top of the piston, the internals were all fine, that was at 176 hrs. I would own them again, but would just use beter quality parts than Merc did in the valve train. A few xtra bucks will save you in the long run. Have someone do a leak down test on any you are looking at and you should be fine.