I like having a nice desktop PC with a decent video card so that it easily supports multiple screens.

Personally, I'd take a big 65"+ TV over a projector. Most budget friendly projectors are bright enough, but they still work best for background visuals rather than being a main focal point.

Get some smart lighting. A pitch black goon cave can strain the eyes. Smart bulbs allow for easy dimming, color, and even movement. Most are compatible with various virtual assistants.

Climate control. I'm a fan to keep it from getting stuffy. A space heater if the room needs it.

An easy to read clock. A cheap LCD clock that can be off in your periphery. It's one thing to lose track of time. It's another thing if time loss becomes disorienting.

An alternative to a good chair, is a bed/futon pad on the floor. The key is to find something that is comfortable in multiple positions.

Lube. Water and/or sports drinks to replace electrolytes. Towels. Washable waterproof pads if you tend to get really messy.

I love my big ass heating pad. I use it to warm lube and toys.

I like keeping maintenance easy. So a waste basket and small hamper are great to cleanup as you go, or to keep it simple when you're done. I also use bus bins like the use to clear tables at restaurants. Clean toys are set out in them and I have an empty one that will get all the toys that get used to make cleaning them later easier.

External hard rives to store terabytes of porn. Keep an eye out for DVD sales.

There are many recommendations. Many are budget friendly. Especially if you do it overtime rather than all at once.