"From my dad's perspective, he wants a boat that will be very reliable and will not cost a fortune to maintain."
I'm not entirely sure these is such a thing
Most of the things that cost more to maintain on an I/O aren't going to break down for another 5-10 years. Until then, I doubt you'd notice a maintenance difference. If it's a Mercruiser, the only nastiness I can think of that occurs on a regular basis is replacing that stupid impeller. It's in the drive leg and requires you to remove the lower unit to get at it. Volvo put it's impeller in a sane location and doesn't have that issue. Gimbal Bearings, U-Joints, drive bellows, and that sort of thing are hard to downright brutal to work on but like I mentioned earlier they don't break down often. Enjoy your dad's I/O.