Get estimates for both geo and air/air. Both can have tax credits if you get the right equipment. Pretty sure the air to air credit maxes out at $1500 but the geo tax credit has no top limit so the 29k cost will get you nearly 9k back on taxes. The air to air unit will still need all the ductwork so you may be surprised when you get the quotes.
Remember they are not just swapping in a replacement unit. They are also sizing the unit, sizing, designing and installing the ductwork, and on the geo unit sizing/designing/installing the loops, pumps etc. Don't underestimate the time it takes to design/size these right. And avoid contractors who just use quick rules of thumb to design your install.
As far as DIY, I'm a firm believer that if a person is willing to spend enough time and effort studying and learning he can DIY most things - but a geo system will really be up there when it comes to studying. They are very complicated systems. Every thing is intertwined and seems to affect everything else. As Beenthere alluded too you don't just do a heat load analysis and then pick say a 2 1/2 to unit because the unit performance depends on your ground temps, loop setup etc.
If you hire a good pro, you should get a good install of a good system that will save you a lot of money and the government will basically pay you the cost difference between doing it yourself and hiring the pro.
If you do it yourself you're going to have to figure out how to calculate your heating/cooling loads, how to select the right size unit, how to deal with the difference in the heating/cooling loads to avoid poor dehumidification in the summer or excess use of backup heat in the winter, how to design your ductwork so that it matches your heat pump and you get the right airflow and btus to each room, how to design your ground loop, selecting the right pump, selecting the right antifreeze for the loops etc etc. Then if you've done that all right you will have a good system that will save you money, but there is definitely more risk and no one to call if something isn't working right.
I think you can still get the tax credit if you DIY but consult with a tax pro to make sure. The credit will be smaller of course since your costs are less and it may be more difficult to get the credit applied to all your expenses associated with the install. If you hire it done you end up with one receipt from a pro that says Energystar model xxxx geo install - $29K and if the model number is on the approved list then an auditor has little to question. If you DIY then you have a pile of receipts for things like ductwork etc. and an auditor might try to limit your credit to the geo heat pump unit only. I haven't found any clear regulation that describes what components of an HVAC system are included in the geo tax credit. So make sure you check with a tax pro about the impact of DIY on the tax credit.
Since you have the trencher, maybe the pro will let you do that part of the work to save some money. I'm not sure I understand why 300ft is your limit on willingness to trench though. That doesn't seem like much trench for a geo system. I was thinking 500 ft per ton wasn't unusual but that's a fuzzy memory.
Whatever you do air/air or geo, DIY or pro, if you are counting on the tax credit make very sure the unit actually qualifies for the credit.