I would probably post a thread on the trading sub-forum here, as well as on Bloomilk which is also an amazing site. A lot of people (sadly) are getting out of the game...I'm sure they'll have a bunch of stuff to sell to you.
As for completeness, chances are those who DO have a complete set and are getting out of the game will most likely not want to sell it all away...probably keep a few pieces here and there. So chances are you'll have to multiple deals in order to get a complete set.
Of course, there's always those "chase" pieces that are outrageously hard to find that exist in all games -- such as in MTG, the legendary "Black Lotus" or "Mox Ruby". In the case of SWM it's Boba Fett Bounty Hunter, Cad Bane, and a bunch of sith lords and others as well. However, if you're willing to pay retail, it shouldn't be hard to buy the majority of the SWM complete set off of various people either on here or on Bloomilk, and then just pay retail for the "hard-to-find" figs off of online dealer stores such as T&T, CSI, etc.
As for an estimate...that's a tough one. Even if someone was willing to sell their complete collection, they would have doubles and multiples of many of their figures. Prices have deflated a bit since the demise of the game so I would guess ~2000, probably more (that's probably a low ball estimate...each 60-fig set has at least 10 VR [total of 10 sets] and the 40-fig sets have 8 VR each [total of 6 sets]. and a lot of those very rares are worth $20 or more. That alone adds close to 2000.)
Anyways, hope that helps a bit!