The ground rent will depend on the year the property was built and the amount of land on the whole development - anything in the range you quote is average depending on the kind of development it is!
As for the number of years left on the lease, this will depend on your mortgage company (as they will have rules on what they consider to be a suitable security for the debt) and, to a certain extent, your own age at the time the mortgage would run out vs the number of years left on the lease.
Just think, if the number of years on the lease runs out you just have to hand it back to the freeholder - no chance to sell it or anything! The fewer years left, the cheaper (relatively) the purchase price should be.