Welcome, John!
TYF is a great resource, you'll find a wealth of information here.
The factory plugs are AC Delco Iridium-tipped plugs, and have a 100K mile service interval. Having owned several newer GM vehicles, I can safely say that while it might make you cringe coming from a Toyota (just traded a mildly built 100 series Land Cruiser for my Tahoe), they really do last that long. Still, I do a tune up and maintenance baseline on every used vehicle I buy, and I throw fresh plugs in there for two reasons:
1. It allows me to get a quick peek into the overall health of the motor by seeing if any of the plugs are overly fouled or out of whack, and
2. It makes me feel better.
RockAuto.com usually has the best price on the OEM Delco plugs, and sometimes they run rebates to get a few bucks back if you purchase a full set of 8.
As for the tranny cooler, they are great factory option but I would consider your climate and how you plan on using the truck before you add one, simply because you're introducing new potential failure points that will bleed out the transmission in short order. If you live in a temperate climate and don't tow much, then personally I would skip it and just make sure to use a high quality synthetic fluid. AmsOil ATF is great stuff. I know that GM has the coolers as an option from the factory, but I haven't a clue as to the part number or if the cooler is available as an add-on after the fact. Your local dealership should be able to help you with that.
I'm looking into leveling kits too. CCM seems to be well regarded, as does the Rough Country kit, which is for the front only and retails for less than $100 ($89 shipped on Amazon last time I looked). I'm seriously considering that one for the price, and it looks relatively simple to install with a buddy in a couple of hours.
Congrats on the new truck!
Edit: Yep, Rockauto is running a promotion on AC Delco Iridium plugs right now - $2 off per plug via a mail in rebate. That cuts the cost of the plugs nearly in half. No-brainer.
