Exactly what RS25.com just said. Open vs. Closed deck is simply the design of the block around the outside of cylinders. If you find an EJ20 WRX engine from Japan, and you know the year, you can find out if it is OPEN or CLOSED deck.
IF the engine is a 1997 Model or Later, it will probably be an OPEN deck, meaning it can't handle as much boost as a closed deck because the cylinder walls are NOT reinforced.
As far as the swap, just make sure you get everything in the package you have found. If you can, get the JDM ECU that comes with the engine, get the tranny if you can as well (JDM tranny's seem to hold up better than do ours). Just remember, Subaru's are like Lego's, nearly any part from any model can fit into the any other Subaru, including JDM models. This is not to say it won't take lots of work.
All you have to do is ask the person/company your buying the engine from what year the engine is, what it comes with, and ask if the engine has had a compression check (this determines if the engine has a problem with the internals, i.e. pistons).
There's lots of information on the internet about performing JDM swaps into US Subaru's, you just need to do some research to find all of it out. I will direct you to i-club.com because the website has been around a lot longer and has more information. If you do a search over there, your bound to come up with lots of info.
Graham