Do your research on geothermal as there are a lot of conflicts out there and there is no point in spending money if you can spend it on proven, well working efficient systems. sydthebeat has given you most answers already. Any company you ring are going to explain it differently to you, if you decide to shop around ring them but once again without a site visit quotes are no good. And syd is right you need a heat recovery aswell. buyer beware. Google it and if you ring companies know what you ask, there are some good companies out there. No harm in asking either if they have installed projects already, and if you can see it work, ask the people that have had it installed if you visit them. All the alternative heating systems from geo to pallet to log boilers they are as good as the people that sell them. If they have bad knowledge, always ask if you can see a system working or even two whatever you decide on.
You always hear the bad of systems, this will give systems a bad reputation that does not mean systems are bad. It just means that whomever sold it and installed it have done a bad job. Do your research internet is great.