Alistair, one of my F3 bodies does exactly what you describe. I have the vertical lines but not the horizontal lines.

I have also seen another F3 with the exact same issue.

I don't have a problem with it at all as I generally only work out exactly where the + and - signs come on, then look at the top of the camera for a quick double check.

I bought my camera new, it is I think, built in 1983/4 and it has done thousands of rolls of film.

The problem started about 5 years ago from memory, hasn't gotten worse nor better.

If the camera works alright and the price is correct, then it's your call, I don't think it will worsen overnight.