There are a variety of different types and forms of Tankless Water Heaters, and if propane is the route you would prefer to go, there are a few great products out there, which I'm sure would meet your budget and/or size requirements. Just be sure to keep an eye on their capacity and 'outflow' details, to ensure they meet the amount of water you intend to use. This is good reference on
Tankless Water Heaters which would be a good starting point to getting a perspective on the whole idea of water heating in general, and of course, the tankless options in general.
I have a lot of personal experience in this area, so if you have any questions, don't be shy to get in touch, and will do my best to give you any experience or perspective I have that may be helpful.