first, i've read all the gshp and geo thermal threads on this forum, and still have a few questions.
i've just moved a house on whidbey island, WA. i have to trench about 400' for power, and another 400' for water. as long as i'm trenching, i'd like to go deeper to set horizontal loops for a 2.5 ton gshp. i've already done the manual j calculations for the finished (remodeled and reinsulated) house, this seems appropriate. the trench lengths are about right for that sized system and my soil type. now for the tricky part. pulling this project together financialy has been a stretch. i've emptied my retirement acounts, run up the heloc, and will be maxing out my credit cards, to get this house certified for occupancy. at that point, i'll be able to get a standard mortgage, and borrow enough to pay all that off and free up the funds to remodel.
what that means, is i need to do this in 2 stages. the loop, which i'll do myself, and the gshp install, and startup, which i'll contract out.
ok, enough back story, now the questions:

if i setup a Dx loop, what size/rating of copper pipe would i use?

and

would i need a specific Dx gshp, i.e. viking, or could i use a standard air to air hp, and hook it up to my loop instead of an outdoor heat exchanger?

everywhere i've looked, i find the "gshp's are NOT a DIY project" answer. this seems silly. i'm a fairly handy guy who's not intimidated by a little math. i can sweat a pipe at least as well as the next guy. i'm just as capable of pressure testing. so the 3rd, bonus question is: is there something special that needs to happen when setting up the loop beyound the capabitilties of your average amateur?