ArjanB : the idea would be to do the preliminaries online. Last year, the preliminaries were really poor (I heard some being cancelled in NL, in BE we had exactly ... one!) ... That's why I would prefer online, the finals should be live.
Dirkdebeule & Awaclus : I agree with Awaclus here. 3 player is nice, but the real strategy of a dominion player is more shown in heads up. To quote one of my friends : "witch on the board, no reaction, no gardens, I don't have 5/2, the other two do ... I'm screwed" - he got $4+ after 10 turns ...).
As for the number of sets needed, in BE we did a very simple rule, which can be easily extended. Take one box of each extension you have. Suppose we have 4, that gives us about 100 cards. Take 10 at random cards for each table (with young witch as an exception). You can set up 10 games this way.