When you reach a fairly large size of heat pump you should also consider having two smaller ones. They are fairly reliable but repairs can be a problem. You also would have the option of using one on its own in spring and autumn.
The collector is much less expensive if you organize it yourself. It involves getting pipe and sand onsite before hand and a digger for a day plus. Also if you cannot do it yourself you need a fairly skilled person who can bring the pipes into the house and connect up. There are always problems with plumbing connections, in Ireland pipes are BS sizes and generally appliances are metric. Also connecting large diameter plastic pipe to the smaller bore header pipes need special fittings so it pays to research every connection beforehand. You should also have lots of valves, flow meters and temperature sensors to make sure everything is working properly. It is better to buy a purge pump and leave permanently plumbed in because the pipes tend to expand a little and lose pressure for a period. Plastic pipes act as a pressure vessel for short term heating and cooling. I only have an overground collector system but I think I would use long narrow trenches if putting the pipes underground. If the ground is rocky it might be easier to have vertical collector. It would require a well boring contractor and special pipe arrangements are needed so plenty of research is required.
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