If you decide to go the hot tub heater route, what you're looking for is known as a spa pack. They can either be bought with just the heater and control unit or can also include a pump. I had to replace the spa pack on my hot tub last year, and I think it was around $700 for the pump, heater, and control unit. Do a search on Spa Pack and you should get several results.

Some things to keep in mind: Unless you have 220V power, you'll want to get a spa pack that is wired for 110V. You will want to wire it into a GFI circuit. An electrician can probably expand on this.

Most spa packs with pumps come with a 2 speed pump, the lower speed is for circulating the water when the temp drops below the set level, and the high speed is for when the spa is in use. Some control units are set up for 2 pumps, 1 a low speed pump and the second a high speed pump. For your application I doubt you would ever want to use the high speed. You might be better off getting a spa pack with only a low speed pump.

In hot tubs, the pump(s) are designed to plug into the control unit, for power and to control the on/off. Not sure if the existing pump you have could be wired to plug into the control unit. Also something an electrician can answer better than I can.

Finally, depending on whether you get an air switch control unit or electronic control unit, you will need to make sure the spa pack comes with the spa side controls, otherwise you may have to buy them seperately.

Hope this helps,
Don