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...the only forced air house I ever rented used an electric water heater.
Anyway, here are all the tips I can think of.
For water: tankless are good, but a heater with a super-insulated mini-tank in it is better because it is more responsive. Of course insulate any pipes that have not been.
There are some valves and T connectors you would need to install if you wanted to do a solar preheat tank later. They aren't that expensive and if you have to rip up your plumbing anyway it would be better to get them put in now rather than later.
Keep your old tank as it might be handy in cobbling together DIY solar. If you have storage space for it, that is.
For space heating: If your ductwork is well insulated and doesn't leak it is an asset. I don't know much about buying forced air heaters but you might just want to look for one that can be grafted into the current duct system. However, if you take another route, see if you can use the old ducts to circulate the air from the roof of the top floor down to the floorboard of the bottom floor. This reduces heat leakage dramatically. Also it might work just as well to allow heat to rise through grates in the bottom floor to the top (depending on many factors.) This is more efficient than forcing the air directly up to the top floor.
In general, circulate air within rooms with slow, efficient fans, like ceiling fans. This actually helps a lot.
Get a clean humidifier and control humidity during the winter. You can get a forced air heater with one built-in, but it is easy to forget that way to keep it clean. Dry air is bad for your health (various reasons) and on top of that it feels cooler, which causes you to crank up the heat past the 68 degree mark which is recommended. One way I take care of this is not to run the shower fan during the winter. It gets pretty sauna-like but once the bathroom door is open everything dries up quick enough to prevent mold, and adds moisture to the air. A $30 hydrostat can be bought at any decent hardware store to keep an eye on things and decide whether showers alone will do the trick.
A good air purifier might allow you to let the house breathe less without the air filling up with crap, as well as function as a fan for air circulation.
Redo your south facing windows with high admittance superglass to let solar heat in during the daytime (2 glass plies with polymer between) and awnings to shade them during the summer. Though a tree that drops it's leaves in winter works pretty darn well if you have it. You can also add additional attic-based or window-hung solar air heaters pretty easily/cheaply as a DIY project. That cuts down on your daytime load.
Most of all, though, do NOT oversize your heating system. You want to avoid it turning on and off too often, since it wastes fuel if not run for long stretches at a time. If you end up with a system that you think might be a little bit too large, add a few thermal masses (large black metal tanks of water.) Those are really only a challenge in figuring out where to fit them amongst the furniture. :-)
There are also tricks that can be played with chemical heat storage if you have enough south facing windows to overheat during the day. However Gaubers Salt tanks eventually stratify so they are not maintenance free. I heard they were working on a 68 degree state transition chemical to be put in "thermal drywall" but I don't think a product is available yet.
If you have a lot of cash to burn, consider a geothermal heat pump. The coils only have to be dug below the frost line, not horribly deep, 5 feet or so depending. Don't get the impression that geothermal implies a 50-foot-deep borehole. Air geothermal is expensive to install but works both for heating and cooling.
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