As a self builder, you would be eligible for the Domestic RHI
If you try to do the self install route you won't be eligible for the RHI, as it won't be an MCS certifed insatllation.
Seeing as a GSHP pays nearly 3 times what an ASHP does over the 7 years under RHI, on a new build a GSHP is a no-brainer, designed properly (in conjnuction with designing the heating system) it offers a far better return on investment and it will also cost you less to run than an ASHP.
Seems like you need professional advice this time round as the game has changed. There are some many mistakes to be be made with GSHP's that's why the RHI regulations REQUIRE you to use an MCS registered company. No if's, no buts, not shortcuts, it has to be done properly.
Ask anyone that has tried to do even a SolarPV system themselves and they'll tell you that the amount of information and variables that you need to learn and consider, that for the next time and for anyone else that asks their advice is simple - use an MCS certified installer !!!
If the people you spoke to couldn't show you either the return on a GSHP or the comparison with an ASHP then you're talking to the wrong people.
This is bread and butter stuff for us.