The tr909 is known for its snappy snares, its hihats,and its claps. The tr8o8 is known for its low kickdrum sounds (think Run-DMC, Beastie Boys). The Tr707 will give you the same basic sounds as a tr909, but lacks the great features of the 909. It is also a hell of a lot easier to find! If it is a 909 you want expect to pay at least 1000 unless you find a deal. A 707 you can get for around 300 bucks. Whatever you decide, GOOD LUCK!
PS If you want a newer drum machine try the MC505 or Boss (Roland) dr202!