The heat recovery from the drains will involve allot of custom manufactured parts and allot of labor to install for an unknown amount of heat recovery.
The furnace idea can be done but a heat exchanger in the furnace will reduce airflow quite a bit and also increase furnace run times as some of the heat is now being sapped into the water exchanger and then reducing airflow also effects comfort in the house.
Both of those pose allot more issues of install and down the road than a hole in the roof for a flue pipe.
Gas waterheaters are $100ish more than electric but they also need a flue pipe and gas run to them if you don't have it there. But the operating cost of gas is far far lower than electric and you will recover the install costs pretty quickly.
If its on an outside wall you can run the exhaust out the wall in many cases. More common up north as most houses there are sided instead of bricked and if you have a basement its even easier.
In your application a standard electric unit would be the easiest solution and a gas unit would be the most efficient. If water usage isn't that great then a gas tankless would work if on an exterior wall or use a condensing version that uses PVC venting.
IMO the non condensing versions arent worth buying unless its outside or on an outside wall as the venting flue pipe is cost prohibitive.
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