Having all the sets:
First should be normal Dominion
Second should be Intrigue if you play with 5-6 players; Seaside otherwise
Third Prosperity if you got Seaside; Seaside if you got Intrigue
Fourth Intrigue if you got Seaside first; Otherwise Prosperity
Fifth Hinterlands .. it's a good set but not a beginners set at all.
Then would get the small sets Cornucopia then Alchemy.
Euro Game Card Sleeves (100 Soft Sleeves/Pack - 59 MM X 92 MM) These are the one you want. I bought them for the Dominion Intrigue I just bought. The only cards that don't fit in their slots any more for me are Bronze and Silver. For the ones that don't fit, I placed them in one the bags that the card sleeves came in and put it in the middle of the box.
It depends on the skill level of the person. In general if you are able to read the cards and can form basic strategy in your head you should do well. I know someone as young as 11 playing but again it depends on what games they have experience with.
If you get Intrigue you get a whole second set of treasure and Victory cards, so you would never run out, no matter how many play. The only limitation to players is really time the game takes and number of action card stacks available to buy. You can play with five or even six with just the basic game, but it tends to slow down waiting for the others, so you're better off to split tables. Even with the basic game you have plenty of action cards to provide a full game at two tables. If you'd rather play with six at one table, add an extra stack of action cards so they don't run out too soon. With an expansion you're golden for maybe four tables without cramping their style. I picture four tables, and when they all finish a game, they each pass a couple of stacks to the next table. Now THAT would be interesting. The game comes with enough action cards to provide a lot of variation, and each expansion (except Cornucopia and Alchemy) add at least another 25 so the options are endless. I just advise that for beginning players you start with one stack of cards costing 2 copper, two stacks costing 3, 3 stacks costing 4 and 5 each, and one stack costing 6. Then when you get good at it, you can go for random selection. I wish I could choose between expansions but each has a different focus, and adds an interesting twist.
2 players is the minimum. If you play with 2 then you start with fewer Provinces. The game is more dynamic with more players but still fun with two. If you want to expand beyond 4 players (ie. 5 or 6) you need to add Dominion: Intrigue to get additional money and victory cards to expand the game.
I find Dominion to have a much higher replay value. There are millions of combinations of cards to play with. Meaning that you have to devise a new strategy with every game... it is lots of fun. Only downside is the game doesnt really pick up until you get a couple expansions. It's not hard to become a master at the base set. If you give your kids this Base Dominion set, they will be begging for expansions to this in no time at all. It can get a little costly, but totally worth it. Awesome game.
I have played both 7 wonders and Dominion. However, I haven't played any 7W expansions and own almost all of the Dominion ones, so take that into consideration. .