Ed, what will your boating activities be? Skiing, wakeboarding,
tubing, or just cruising?
I/O drive systems are actually the most complicated type (full outboard
and full inboard are much simpler). However, they do tend to run
quieter, cleaner and more efficiently than older outboards, depending
on how you use them. I/O's also can be had with more power if you need
it -- which you will need at least because of the extra weight! A rule
of thumb is to add up to an extra 50% to an outboard HP rating to get
an indication of the equivalent performance in an I/O (e.g. a 90HP
outboard to a 130HP I/O). The main difference here is that most older
outboards are two-stroke (lots of power and low-end torque for their
weight), while I/O's have four-stroke, using conversions from
automotive engines.
How much older of a boat are you considering? Newer outboards have
apparently greatly improved in all of the negative aspects that I
mentioned above, but I don't have details on how and when most
improvements became available. Maybe someone else can horn in here.
Let's see... it is mid-June. The market for used boats is rapidly
dimishing if your main activities include the ones needing a fair bit
of power (just about anything involving towing). If you just want
basic cruising, take your time. In the size range that you are
considering, especially the lower end (17-19 feet), I/O engines will
come mainly in two sizes: 3L 4-cylinder 130HP and 4.3L V6 175+ HP.
Ignore anything that isn't a GMC engine (some failed experiments were
done with Ford and Chrysler engines in the 80's). If you are just
cruising, the 3L is fine. If towing, it's underpowered, which is why
the used 4.3L ones get snapped up quickly, while used 3.0L boats
languish for sale forever. Around 20 feet and up, you might find some
with larger V8 engines. These will all have tonnes of power, at great
expense in fuel consumption. Shop as you like, but I would avoid a mid
or large sized engine with the Mercury Alpha drive -- the Bravo drives
are apparently much better. I had a 1991 OMC Cobra drive that was
flawless.
This is all just from what I picked up while shopping for and operating
my boat for several years. I don't have any market research with hard
numbers, so take my free advice at your own risk.

See my previous
advice to another buyer here:
http://groups.google.ca/group/can.rec.boating/browse_frm/thread/1dfba4...94c16a7
Ian.
>> Stay informed about: Tri hulls, and boats in general