A lot of site I have seen just plumb a hot water line to the intake of the solution pump.
I would just plumb a hot water line from your site water heater to the inlet where the water heater is hooked up now, there will be time delays but if your wash a few cars it should warm up the line. Insulating the line will help also.
I personally would not stick that much cash into a water heater if you are going to replace the machine in spring.
You could run with out it also for the winter, depending on your cleaning issues, if you are just rinsing salt off the cars like everyone in the northern half of the US, you can probally get away with out warm
soap. It WILL effect the
soap effectivenss, but you may be able to go one tip larger to compensate for a few months.
Hope this helps
Regards
Mike