Jesse has got it right.
Even a 2HT will fit the bill.
Caution on older heaters, on the standard copper coil little giants (which most people run) there is a 600 psi pressure relief on them for a reason, or at least there should be, I won't sell heaters without a pressure relief.
They are "rated" for a 1000 psi max spike, but clearly state 600 psi sustained.
The one we sell the most of is the "high test" copper coil, it's what I call the carpet cleaners heater, it can handle sustained 1000 psi, and is good for the guy wanting to do tile work with his spinner, it also has thicker walled coils that will last longer.
Because your lil' steamway unit can only make so much pressure, you'll probably be fine with a standard heater, so if you see a used one cheap, grab it.