Looking at my desk, I can see a few things that I'd be lost without:
1. Make-up brushes for drybrushing and a lot of cheap synthetic brushes for metallics, washes, and the first 80% of a model. Vince explains it very well:
https://youtu.be/YTm_LgpOA0Y
2. Good lighting. I use two daylight lamps, but you may prefer a single large overhead lamp.
3. Magnifying lenses, depending on your age or eye-sight. I was very surprised at how much difference they made to me.
As far as thinning paints goes, I use a wet palette, tap water, and flow improver for brush painting. I use a 50:50 mix of airbrush thinner and flow improver for airbrushing - except for vallejo surface primer, to which I just add water. Everything I spray is *very* thin - more thinner than paint.