Short answer - avoid it! You say Freeview box (which usually mean just a receiver); but the Thomson is a PVR - is that really what you want? Anyway, the Thomson has Top Up TV software which has some rather bad write-ups. If it's on e-Bay cheap, there's a good reason.
In fact, you'll be lucky to find a PVR in reasonable nick for £50. The cheapest on Argos were about £100 the last time I looked. However, if you would be happy with a receiver, not a recorder, then £50should get you a reliable machine, brand new.
PS Re the Thomson, read this -
thomson dti6300 pvr - any good?