If you're going to be doing extensive driving, you might want to consider doing that TB then - may as well do waterpump and thermostat while you're at it. Rough service interval is 100k km's/5 years. It's not an interference engine, but snapping it will leave you stranded.
5w-40, 10w-40, doesn't really matter. Though, if you won't be going anywhere near cold temps, i'd go a 10w since it'd be slightly thicker at lower temps, reducing the risk of oil slipping through worn/thin seals. Whatever floats your boat
O/D(overdrive) is just 4th gear. If you turn it off, it'll never shift into 4th - even when doing freeway speeds. I'd leave it on unless you're doing city driving and don't go past 70-80km/h often.
If it wasn't fitted with cruise from factory, vacuum ports will already be blocked off
Yes, these engines do have an idle air control(IAC) valve. It;s that little hole on the bottom of the TB before the butterfly valve. Give that a good spray/soak with Seafoam(or TB/carby cleaner) and scrub with an old toothbrush. You can clean the butterfly valve too. Not really carbon, just oil vapour(and maybe a bit of dust) that has condensed there from the PCV vent coming off the valve cover into intake manifold.