Been here, done this ... Exterior venting is mandatory for your appliances. Make sure it is clear from any upstairs windows so your off-gasses do not come back inside. That means your ducting will be specified by code.
An exterior mounted tankless water heater might be your best bet. Eliminates the need for exterior venting from the inside -- but you still have to follow code to avoid re-entry of exhaust gasses upstairs. Also, snow accumulation is a concern because you cannot let the unit get buried! ... So, you might end up using an interior mounted tankless with exterior venting and a way to conceal it for aesthetic reasons.
The furnace and its location does present problems. Obviously exterior venting is needed but it also needs a closet to isolate it. The closet door vents are determined by how much combustion air your unit requires. There is a standard formula for this. If your house is extremely weather tight, where will that air come from? Also need to consider the HVAC ducting, the size/design, and where it will go in your downstairs ceiling ...
And while we are dancing around permitting, have you factored the need for additional electricity? Your pre-wiring will be crucial to supply the demands ... and let them pay for the added usage. And, I wonder if you are wanting to add a separate meter for power? If so, you will need a sub-panel, permits, inspections, etc.
Hope these thoughts help ...
Last edited by CallMeVilla; 07-10-2014 at 08:46 PM.