The Koni-Omega TLR's appear from time to time on E-Bay. I think there's one listed now under Medium Format. Ive always been tempted to get one, especially because the Koni backs I already have will fit. Since they usually sell for around $300, I have managed to resist temptation! I figure the money would be better spent on something newer such as a Fuji 6 x 7 rangefinder.
I just checked E-Bay; there's currently no Koni-Omegaflex list in Medium Format. The listing I saw recently must have just expired.
Ailsa, I have used both the Mamiya 330f and Rollei TLR's. From the point of view of getting the job done (i.e. just taking good pictures), the simplicity of the Rollei's is hard to beat. Keeping it simple has an interesting side effect: you forget the camera and concentrate on the photography.
My best pictures have been taken with my old Rolleicord III (and I own a rather wide range of equipment: Mamiya 7II, Horseman VHR, Crown Graphic, several folders). Recently I succumbed to the siren call of a Rolleicord Vb (and it had grey leatherette finish, which was the clincher), but it's really nothing more than a fancy III! You can pick up a 'cord III for less than $200, get it CLA'd, and you'll have a real photographic machine to work with.
It's hard to transmit one's experience to others, and it doesn't always transfer over well, but here goes ... you can't go wrong with a Rollei TLR, and the simpler the better!
You are getting pre-owned equipment, so it would be a little bit difficult to generalise: a Rolleiflex might have a better reputation than a "Something-flex" made in somewhere else, but if the Rolleiflex had been trashed by a previous owner, its lenses no longer properly synchronised in focal length, etc, it might not be any more satisfactory than the latter.
If you see one at a dealer, try to arrange for a money-back guarantee: that's what I always do:
Seal your cash in an envelope with your contact details etc, and get the shop manager to sign it as well, let him put it in the cash register, the deal is this: if you do not return with the camera within 24 hours, he can open up the envelope and put the money away; but if you return with the camera, you can take the sealed envelope back. You take pictures with it for the day and get them processed and carefully examined, whether it works or not, you still won't be out of pocket.
I have done arrangements with dealers like that and it seems quite acceptable to every one of them; it is perhaps worth a try.