I would look into chairs which are used in 24/7 call centers. In my previous job I priced some of these out, prepare to spend no less than $1500 for something decent with good lumbar support.
As we've learned with bicycles, good stuff doesn't come cheap. Is $300 is your absolute price cap then check into second hand at the good will, or go to Staples and park yourself in a couple chairs. make sure you wheel around the isles and play with all the adjustments to make sure they don't feel too flimsy.