In my opinion, the underground PEX is never a good idea. You are always going to loose some heat into the ground, especially if your plan is to run the pump 24/7 like the outdoor wood boiler people do. It's a better idea to have the boiler inside the building your going to heat.
That being said, A lot of people put boilers in outbuildings with varying results. The better insulated the pipe is, the better your install will work. Making your own insulated pipe can get expensive to. Thermopex is one of the best out there. http://www.badger-pipe.com/
is less expensive but may not be as well insulated.
Don't even think about the 1" stuff unless your house is very small and very well insulated. Work the equations and do the math so you KNOW what size pipe you need. Neglecting this step is a recipe for disaster. Remember, 1" PEX is only 7/8" ID. The crimp on connections for 1" PEX are only 3/4" ID. Normally, you can only push about 70,000BTU's through the 1" stuff, without resorting to extraordinary measures.