Floyd Bed Frame — The Floyd frame can be bought as a queen size, then upgraded to a king later. (https://floydhome.com/products/the-bed-frame)
Parachute Linen Sheets — kind of expensive, but the first set of sheets I've felt excited about using (https://www.parachutehome.com/products/linen-sheet-set)
~5 years into a cheap mattress off of Amazon, but I've been really happy with it so far. I think it was a Wirecutter recommendation: (https://www.amazon.com/Dreamfoam-Bedding-Plush-Pillow-Mattre...)
Before I moved in with my girlfriend, I was sleeping on a twin mattress on the floor! (With a small rug underneath it). So I suppose anything would have been a step up.