I second what Nick said,
I own 2 boats, an aluminium side console fishing boat and a fibreglass
speedboat. Whoever tells you fibreglass is no good is offering you useless
unfounded advice. They both have their pros and cons. Weigh both sides for
yourself. Speak to a dealer who sells/specializes in both. From experience,
maintenance and durability standpoints, I'll never buy aluminium again.
Heres why. If you plan on never hitting anything while underway then
metal is a safe bet (these same people also bet they will never be involved
in a car accident!). Otherwise, you'll end up like I did, with a very large
dent in the hull. Yes, It happens to even the best of us. Hitting a tree
stump at 25mph can cause severe damage to aluminium that is not easily
repaired, whereas fibreglass can usually be spot repaired in this type of
circumstance (if there is even any damage more then a scratch for that
matter). Poor welds can deteriorate and become weak, leading in turn to
excessive flex and weakened hull integrity. In severe cases, seams can come
apart from vibration, riveted hulls are worse. Inevitably, this in turn
leads to leaks. Another common issue is galvanic corrosion. It tends to be
greater on aluminium boats then fibreglass due to the metal to water to
metal contact. Some rattle and are irrefutably nosier then fibreglass
because of the nature of metals conductivity to noise, save maybe the more
expensive fishing rigs (which can be rather quiet), but that's not what you
are looking for. Aluminium is not easy to keep shiny, fibreglass is not any
easier.
In some respects being lighter then fibreglass is a benefit. They need
less power to achieve the same performance as a comparably sized glass hull
so in turn will probably save $ on fuel (smaller motor). But this can also
be a bit of an annoyance being lighter. If you plan on operating in anything
more then 1-2 foot seas you'll see what I mean.
Fibreglass is heavier so the boat may not toss around in heavier seas,
but, more weight means more motor which equals more $ all around. Osmosis
can be an issue. Gelcoats fade and scratch and require continual upkeep
(yearly) to maintain that glossy shine.
As for outboard vs. I/O? Personally as one who has owned and worked on
both, The ease of outboard maintenance compared to I/O is unparalleled. To
rebuild my powerhead I can have it removed and ready for the machine shop in
1 hour, I challenge even the best marine mechanic to match that with an I/O.
It is quite an ordeal, and not something you can do in your own garage.
General up keep and maintenance is easier to do on an outboard in my
opinion, but you need to know what you are doing. I/O's are basically
automotive engines converted for marine use which in itself has pros and
cons. Winterizing is easier on an outboard and pound for pound delivers more
torque and horsepower the average I/O when hp to weight ratio is considered.
As mentioned earlier, they have their downside too though. They are
nosier and can be finicky. They're confusing and messy to some when mixing
oil with gas and if you forget the oil.....guess what, bye bye motor. Oil
injection is not any safer, I have heard of and seen countless cases where
the injection pump failed and the engine was oil starved causing severe
cylinder scoring. These are just a few of the tidbits of info to help you
get started in making your decision.
If you are new to boating, and considering buying an old boat, before
you lay your hard earned $ for a downpayment- Do yourself a favour....find a
local marine mechanic, pay him to do a once over on your perspective
purchase. There are countless hidden areas not discussed here that can turn
into financial disasters. Worse still, is finding out one fine evening when
out with the family out on the water and miles from shore. The sun sets....
the motor goes quiet.... and you discover then and there, you have a
problem. Hope this helps.

regards
Bryan