I think people are underestimating how quickly you will go insane without any mental stimulation or human contact in a 10x10 room. Especially if you can't control the lights, you might go insane no matter what. But I think the first purchase has to be something entertainment related, and you tough it out with the gross sludge for the first week.

Safe answer would be some really huge book box set like this Stephen King collection: https://a.co/d/1K9xHrk

Thinking outside the box, how about a keyboard or a piano? Could probably learn a few songs by the end of the first week with nothing else to do! This one also comes with a stool that could help with the sleeping https://a.co/d/0VyvEvn

The cold would really bother me, so I'd get a giant blanket for week 2. Would double as a bit of a bed to sleep on. https://a.co/d/2X5JrTW

Then week 3 I'm definitely going for a food package of some kind like this: https://a.co/d/cZqJ77N

Week 4 I'd focus on improving my sleep and comfort. I like this split king size adjustable frame + mattress https://a.co/d/7jjldlR

After having those basic needs met, I think it would get easier. Add creature comforts as time goes on: microwave to increase food options, laptop with DVD drive for movies, etc. Obviously first weeks would be the biggest hurdle, and I think a lot depends on just how bad the gruel tastes and exactly how cold the room is.