That's a great deal and a clean ski! You're really going to like it.
Does the trailer have straps in the back to hold the ski down?
How's the engine bay?
For the stock ski, you just need to run 87 octane per the owner's manual. I've only run 87. No harm running 91 but it's not necessary. You may want to consider running ethanol-free gas. Pure-gas.org shows where stations are, looks like there are a lot around the DFW area. I am picky about maintenance and keeping things dialed though.
I used to run Pennzoil Premium Plus premix, the second link. The XLF is "extended life formula" but I was never actually able to figure out the difference other than marketing.
I started running Klotz Skicraft Techniplate premix oil, which is about $44/gal on Amazon. No issues or specific reason to switch, I just wanted to move to something a little 'higher quality' once I picked up my new ski. It's also orange instead of blue and maybe won't darken the gas tank plastic color, but I have no proof. I also run the oil at 40:1 because a little extra oil never hurts, and it's not enough change to affect the jetting. 2 vs 2.5% oil. Not trying to start an oil debate though. 50:1 is what is specified in the manual.
Yamalube 2W is also a great choice, $36/gal from Rocky Mountain.
I recommend one of these type of bottles for mixing since it has a cap. I used to use the classic Ratio Rite but this is way easier.
https://www.amazon.com/Maxima-bottle-stroke-engine-premix/dp/B007KPWL5K