dust in the lens is rarely, if ever a problem, unless it is big dust balls, which will reduce contrast, but I have a few different lenese that have got dust in them over the years and has never cause a problem..but it is a very good negotiation point
I purchase a 300mm f/2.8 a couple of years ago and got a real good deal because of a 2mm scratch on the outside edge of the front element and it has been a great lens with no ill effects because of the scratch
A TLR with lens is much better than a TLR without a lens.
As Dave said, it's relatively unlikely the dust would be much of a problem (unless there's also a kitten in there, chasing the dustball), even though you'd want to make a big deal of it as a buyer. In fact, you might discover it's actually on the rear element, and easily cleaned when the camera is opened.
In fact,if the dust DOES reduce the contrast,that will be easy to correct as I never print harder than grade 3 anyway-so I have an absolutely PRISTINE grade 5 filter!
Besides which,some faults can be artistic-which would explain the popularity of certain Russian cameras!
I'll have to check if the camera is still available.