Forced air, radiant heat, and radiators are all heat distribution types. Geothermal will work with all of these. The reason forced air is most common for a new contruction is that you get heating and central A/C with the same ductwork.

Retrofitting geothermal in your case would involve installing a ground loop (vertical or horizontal) most likely unless you have a large pond nearby to tap into. As far as credits, they are minimal. I got a $300 credit for our install but I think that too is being phased out.

You'll need to get some bids to see if the payback is worth it for you. In general, the numbers look more attractive with new contruction versus retrofitting. Good luck.

We installed a 5-ton Waterfurnace heat pump a couple of years ago and are very pleased with it. We do burn some wood too (now only ~4 cords/yr) but only as an adjunct to the GHP.

Our house is ~2200 sq ft (more like 3100 if you include the basement) and is thoroughly renovated from the original 1890s era brick bldg. Our winters here in west Quebec can get as low as -40F and summers usually get up into the high 90sF and sometimes exceed 40C (~105F I think).

Our heat source is a closed-loop heat exchanger in a nearby spring-fed pond, and I was concerned about 2 things: the heat capacity and potential animal damage (muskrats). A heat calculation showed we were on the tight side, despite reasonable flow through the pond, so I enlarged it by ~9 dump-truck loads. I'd have liked to have made it deeper than the 8' where the Slim Jim heat exchanger is located, but was concerned about breaking through the clay into sand. The muskrat problem pushed us to use the Slim Jim and I'm quite happy with it. In the summer, the cold water is sufficient to get the house cool without needing to ruin the compressor.

Here in Canada there are now very good rebate programs -- as much as $7000 -- but we missed them due to timing. I have no regrets though: in the 1st year, our energy cost was cut in half, and the house is much more comfortable. A GHP circulates the air continuously and only exchanges warm air instead od adding bursts of hot air. It may be possible to retrofit to use your radiators, but if you can install ducting, I'd recommend it. Having AC was an added bonus for us, and our cold pond water means the compressor doesn't have to come on to get things cool so it is "almost free"!

BTW, the costs are coming down but a GHP is still a costly prospect: ours was ~C$25K installed. The good news is that the cost can add that much to the value of the home (unlike conventional furnaces).