TLR's are fun to use, I use them alongside my LF equipment, don't be afraid of the square format, I can think of some great photographers who shot to the format and never cropped, but that's a personal choice (not to crop).
If you're happy with a fixed lens then something like a Yashicamat 124 or 124G is a good first TLR, sometimes you cam=n be lucky and get Rolleiflex cameras at bargain prices as well. If you want inter changeable films them there's the Mamiya C series are excellent, something like a C33 or C330.
Ian