They are mechanically very simple, so the thing most likely to go wrong is that the slow shutter speeds are either slow or completely out of whack. Same as with all lenses, a clean bill of health with regards to no fungus, no big scratching and no terrible cleaning marks are always a plus.
Keep in mind that TLRs are cameras that never really were significantly popular past the 1960s - the Yashica Mat 124G is the exception, but the vast majority of cameras, especially within your budget, are going to be upwards of half a century old and that context is important to take note of.
An original Yashica Mat with a Yashinon lens would fit within your budget and make a decent shooter - a little soft at f/3.5, but the big negative counteracts that problem and it performs well enough for a camera released in 1958. Minolta Autocords are also popular choices, although they have increased in value (as have most other TLRs, in line with the general increase in film camera prices). If you could stretch to a Rolleicord IV, I believe that the IV with a Xenar lens offers a great bang for the buck - and if you don't like it, they hold their value very well.