to answer the question about the scan tools, you can get a Rinda tool for about $500, it will cover all the MEFI's(merc, Volvo, Crusader,etc), the Volvo EGC, Delco distributors, later Merc EFI, MERC 555(will not do Opti). The DDT will cover all the same but it will also do Optimax. The new Merc CDS is a laptop driven program that is around $1200 for the software and harnes up to $7000 for the complete toughbook,DMT2000 multimeter,harness package. You can get one, but unless its plugged into Mercnet(dealer only) once a month, the system will crash and have to be rebooted from the original disc and set up thru Mercnet again(pretty sneaky from Merc). Supposably its doen this way to make sure any problem that involves omissions will be verified that its corrected. Truth be known, its just another move from "Big Brother Brunswick" to controll the dealers and the market.
As far as chosing between I/O and O/B. That might take awhile to explain. I make my living working on I/B and I/o's mainly(I do work on OBs, but not as much). If i were to spec out the I/O as I would want it, i would be as reliable and user freindly as any OB, the only problem is that no one would put that kind of package in an I/O boat for general sale. My choice of I/O package for use around here (coastal SC), would be the Volvo Ocean Series engine package, with the composit drive/transom shield. The engine comes with closed cooling but it doesn't include the manifolds or risers(merc doesn't either).The nutra-salt system would help, but I would like the Crusader cooling system. it includes manifolds and risers(yes risers also). Or either use a Volvo OSI diesel package that is completely closed cooled. So after spending much $$$$$$$$, you would have a decent saltwater package. If you're running in fresh water, its not that big of a deal, you can piece together a Merc or Volvo pakage with a 5.7 and it will last. If you have an older boat, upgrading to Alpha1 gen 2, or Volvo SX/DPX would be a good start. I/O do require more service, and if you're worried about freezing, the closed cooling helps that situation(some things will still need to be drained). That being said, my V has OB power, and thats what I'm going to keep on it, If I had teh money to buy a new OB, I would proabably put a Opti on it( just buy the extended waranty), second choice would be the 175 Zuke. I'm not going to be buying a new motor anytime soon, so I'm looking for either a good solid 2.5 merc, or 3.0 Merc to have ready as stand by for the 2.4 200 I have now. there are a lot of things to consider, mainly if you're working on it yourself or paying someone to do it. Newer stuff doesn't give you much choice. This is an argurement that could go on forever, but for what I do, knowing I have equal access for repairing and parts, and where I use the boat, I prefer the OB.