You do not say what you want the TLR for.
If it is for exploring the concept of using a TLR, and getting used to the waist-finder and other very different attributes of using a TLR, then you might consider some of the lower-end devices. The Ricohflex is a great manual device, and a good place to start. Especially as the prices are very low.
I am not a fan of the Seagull and Lubis as TLRs, but as toys me thinks they're wicked (less so the Seagull I suppose; its just built like a toy).
As Susan and a few have also noted the Mamiya are indeed a great set of kit, especially with lens interchangability, for which they are unique. These too are relatively inexpensive (compared to some German stuff), and give you a lot of options. Make no mistake though, both the 220 and the 330 are BIG BUSES. Last forever though, but not the sort of thing you want to lug all over your Great Wall trek.
The Yashica Mat (culminating in the 124G) is a very viable mid-point. Price wise a bit lower, noncoupled metering, external lens kits (a compromize for sure), and a definite step up from the Ricohflex, Seagull and beloved Lubi.