...I understand prices from Mercury will he sky high...
Prices are a little higher because of the uniqueness of this engine. However, there are marine parts suppliers that can supply parts. They buy the parts from Mecruiser and other vendor and give better prices.
...and the usual corrosion on/in alloy blocks...
I haven't seen any corrosion issues. If anything corrosion would occur because of lack of maintenance.
...Is the starter specific to this engine or do other starters work...
Yes, the starter is specific because it has the standard Mercruiser 2-bolt mounting flange. They are available in a Delco type case. I'm using a 7lb geared starter that I found available.
The water pump is built into the engine block, and does have seal problems with water leaking into the oil sump. The cure is to keep the seals fresh.
Jan
