I grew up with Legos and was thrilled when SW became a license. Unfortunately, I don't have the room for them and they are all boxed up right now. I really wish I had the room for them (the wife won't let me take over the spare bedroom) as the Lego line is one of my favorites.
These things are expensive and I usually try to wait until clearance or sales to pick them up. Got some pretty good deals that way - UCS TIE Interceptor for $25 and the original system Millenium Falcon for $50 at Target and the UCS Yoda for $10 at Walmart.
Also have a complete set of the MINI Line including all of the promotions and the exclusive Kabaya Japanese set (X-wing, Boba Fett's Slave I and the TIE Interceptor that was released before this year's promotion). These I do have displayed as they're small enough to find room for. Shouldn't be too difficult to track down the basic ones, the free promotions might be a little tough but not difficult I would think. As your focus is only OT, the TIE interceptor and TIE Fighter would be the only two free ones you need to track. The Interceptor is still sitting at a TRU near me, let me know if you want me to pick one up for you ($5). The TIE fighter you'll probably have to resort to ebay for, this came and went quick.
As for mini figs, I'll give you the cheapest set with which the character came (I'm assuming you'll want the original costumes, ie, Cantina Han, Farmboy Luke, etc.)
Han Solo, Tatooine Luke, Old Ben - buy the Cantina Set that comes with the landspeeder, dewback and sandtrooper and part of the bar. Kills three of your wants right there, plus you get a Greedo and Sandtrooper to boot.
R2-D2 / C-3PO - either the Escape Pod or Jabba's Entrance (may not be the correct name). Both are under $10. The second set comes with a Bib Fortuna also.
Chewie - AT-ST
Princess Leia - good luck. The only set her ANH costume was released in was the now discontinued original Millenium Falcon (not the new Hoth version sitting on the shelves now)
Darth Vader / Emperor - Final Duel (I or II, I forget). Comes with Vader, Emperor, his chair and the window he looks out during ROTJ.
Stormtrooper - the other Final Duel set. Comes with Jedi Luke, Imperial Officer and a stormtrooper. I bought a bunch of these at KB for 3 / $10. Might still be available. If you want, I'll send you an extra Jedi Luke. Just PM your address and it's yours.