Umm. I could be wrong but Im pretty sure every time you have any modern outboard serviced it would be hooked up to a computer to tell you whats been going on in it. The etec is VERY hi tech as you probably know and you will struggle to fix almost anything yourself that breaks down. even if you do fix it, the computer will probably have to be played with to make the error codes go away. I dont know what model etec you are talking about but I was told by an etec mechanic that they are so tightly tuned, that when they take the spark plugs out to inspect them, they have to put the same plug back in the same cylinder to avoid some kind of malfunction. I have never heard of this before on any engine, but he was the mechanic, not me. if I was in your situation I would be looking at an easy to work on carb 2 stroke.
When I looked at getting an etec I asked about the 300 hour service and the mechanic said that they still should come in to the service centre at 150 hours for a technical service, which was mostly software checks. but still I dont know how you would be able to do anything like this without a dealer.
I also wouldnt be getting too worried about the etec power v 4 stroke. look at the test etec did V the mercury 4 stroke. IF you believe the etec tests are 100 percent impartial which is a big if, the 150 etec beat the 150 merc by about 10 meters over a drag race that looked like about 5km long. Absolutely nothing in it performance wise.
If it were me I'd get a carb 2 stroke that I know I can fix, or any half decent mechanic can fix without any software. Your choice though.