Hi Alex,
Most folks that do this buy an old engine that has not been rebuilt, so the cost of the engine core itself is not that high. That's good news, but in the US the shipping costs then become a significant part of the purchase. And shipping these can take a while (figure 2 months best case from Italy.)
Remember that since you are buying something that's come out of a car that's been scrapped, it's highly variable what the condition will be when it arrives. I bought two from Gabrielle David (aka alfa1750) on e-bay. He was very responsive and the engines came with all the accessories that he promises -- wiring harness, ECU, starter, coils, etc. (even the power steering pump, but I don't think I'll put power steering in my GTjr.

)
In my case it took forever for them to arrive though – over 4 months! -- because of August holidays in Italy and delays in US customs. Note that there are handling charges on the US side that push the shipping costs above the basic rate from Italy. (About $200 more in my case.) And then there’s customs – this kind of importation falls a bit outside of what they are used to, so the questions you get asked seem to vary case-by-case. So, if you go this route, expect it to cost about $900 ~ $1,000 per core and take at least 2 months – not bad (for something exotic) if you’ll do the rebuilding yourself and aren’t in a hurry. I haven’t done the rebuilding (or even torn them down) yet, but I expect the parts costs to be comparable to a typical 2L rebuild (i.e. the parts cost will outweigh what you've spent on the core.)
Other's who have done this report that the parts are fairly easy to find -- in fact some are interchangable with the older 2L parts.
Good luck! (and post your experiences here!)