The motor load for a unit heater this small is unlikey to exceed 1/8th HP. If you were to use a standard safety switch, a 3 pole 30 amp switch would do the trick.
An alternative would be a 3 pole switch such as a Hubbell HBL7810BW. This type of switch is made by others, but that is the only catalog # I have handy. This switch will cost about as much as a safety switch and will probably need to be ordered.
You may not need a disconnect at the heater if the circuit breaker complies with 424.19(B)(1)