I have DQ9, so fact from fiction:
You do not -need- to have 3 other people with a copy of the game. You can easily play through by yourself, with two people, three or four. You can play through with a few people and some generics, which you make in a kind of DQ3 style. The leveling is easier, the spells cost less, and it is generally an easier, more casual DQ game. The multiplayer is indeed local only, the wifi element is only for using DQVC, where you can buy a few randomly selected items each day from an online server. Sometimes there's great stuff, sometimes there's only common stuff, sometimes it's both. Speaking of the multiplayer: when you join up, each player can run around the host's game world separately from the others if they like, effectively letting the group accomplish multiple simple objectives at once. Not story stuff, but still. Me and some local friends have been saved from group wipes several times by being split up when something bad happened to one group, and being able to wait and get revived by the others. You also have the option to get revived back in town.
Fun crafting system that you can use pretty easily and that will keep you ahead of the shops if you are willing to dedicate yourself to tracking some things down, but it's not necessary to do it. You can make a profit on this system if you want to, too. There is a gold bank, so dying doesn't always ruin your money. It's not hard at all to connect to other players or switch between players you are connecting to. Class changing is a hell of a lot easier and really has no chance of screwing you like it did in 3, since you can change back whenever and get all your old stats back. There are almost no random battles unless you are at sea: You can see enemies on the world map and try to avoid them if you want, or run right at them if you want. In addition, if you sneak up on them properly, you may be rewarded with a surprise round. It's... basically every gripe about the DQ series I have ever had effectively solved while still making a good game with a fun narrative and quirky humor.
It is a lot of fun. If you have $35-ish to spare, give it a shot at least.