What is the average cost of a new 525 efi from mercruiser?
What is the average cost of a new xr drive?

After an engine reaches its lifespan of 600 (average) hours, what is the next step? Do you replace with new, or is it worth overhauling the old ones? If replacing with new, do you have to return the old ones as a core charge like a replacement auto engine?

I am looking at a boat that has just been freshened. If i buy it, what should i do when i put the next 300 hours on it?

I'm curious myself.......

How many times can you rebuild an engine and still get good reliable performance?

???

The # of times an engine is rebuilt isn't as important as is the clearances of the bearings and pistons. As long as your crankshaft doesn't need turning, a forged steel unit can be used for a long time in several builds. That assumes of course it passes a magnaflux test. I'm sure at some point the metal fatigue will get the best of it, but that should show up as small cracks when magnafluxed. I personally wouldn't use a crankshaft that had to be turned more than 10/10.

Cylinder walls I've always seen .060 over as kind of the last step. There are many after market blocks that can be bored more than that, but factory stuff 60 is about it in my opinion. That being said, you can rebuild it several times up to .060 as long as the bore is in pretty good shape and boring it to .020 once, .030 next, and then .060. As long as a good cylinder wall surface can be achieved, there is no need to bore any larger than required.

Rods can go either way. If they clearance well, and magnaflux ok, they can continue to be used.

Everything else is pretty much replaced to new when doing a rebuild.