The TLR is your best bet. I, too, have stood right in front of a person, framing, focusing and shooting, and she never knew I was there. Your head is bent down, looking in the finder, and it just looks likle you're fiddling with something.
Good street pictures are made by finding a spot with the right background, framing and focusing and getting the exposure set, then waiting for someone to come along and complete the picture. Robert Doisneau did this quite often to good effect with a Rollei. He would stand there and shoot a whole roll over a period of time. You can see this technique at work in his "Doisneau Paris" book. Perhaps the best street shooter of them all. One piece of advice: Save a little longer. $250 is pretty slim for good MF photography. Oh, and check our the Sekonic L308s. Small, light, accurate digi meter that does reflected, incident and flash, with and without cord.