Survey come up that roof felt requires replacement

Hi all, apologies if this would have been better on housing UK but thought might have a better idea here!

Just in process of buying a 1950s ISH semi (ftb). Survey has shown that roof felt poor state and likely needs replacement. The tiles have a lot of moss and think these also at end of their useful life. This wasn't known when offering. There isn't any damp to speak of and can't see any neighbours with new roofs. Polystyrene possibly covering damage?

Is this something that will be a huge undertaking, is it a case of getting quotes and maybe renegotiate or not bat an eye and go for it knowing it's stood this long? Any ballpark on price to fix or someone done similar?

Thanks!