New to wood boilers. Need advice.

[whitey82nd]

I am interested in building an outdoor wood boiler to supplement my heating bill. I want to build my own system because one I am cheap and two I don't want to invest in a manufactured boiler until I am sure that I want a new system. I have found a homemade boiler on craigslist with everything included. The pumps, firebox, aquastats and draft fan. The only problem is the water tank is made from two propane tank ends with a fire box built into it. I called the builder and he said it held only about 35 gallons of water. The chimmney pipe looks like it goes up the middle. Seems to me he is sending most of his heat up the pipe and only using part of his fire to heat the water. My question is could a guy add an existing tank over the top of this and use the heat from the chimmney pipe to heat more water? I have a 100 gallon propane tank and welding experience and could easily build some fire tubes to extract more of the heat from the exaust. I would eventually like to have about 250 gallons of heated water. Could I add additional water storage in my basement with additional insulated storage tanks or used hot water heaters piped in series? Any advice would be greatly appreciated.