You're mixing up a few details, as far as I can tell.
TX school shooter was bullied at school because of the clothes he wore
He was bullied over his clothing because of how he dressed, not because his family was so poor he couldn't buy clothes (he was bullied for being poor as well, but "poor" doesn't mean "has literally no money"). I also can't find anything that says his truck cost 60k....in fact I can't find
any truck that costs that much, MSRP on a 2022 F-150 is about half that (and new cars are expensive right now).
He had a job at a Wendy's around the time of the shooting, which pays about 350 a week, which with a little saving could easily add up to the cost of the weapons he bought. Additionally, he would have gotten a tax refund a few weeks ago, and as anyone who's worked food service knows, when that's your only job you get most/all of your federal income taxes back in your refund, so he would have gotten a decent amount of cash (I think I got about 2 grand back, but it's been years since I was at the tax bracket so I'm not sure).
I don't find his appearant financial situation at all suspicious.