Many older sets of bells are in need of restoration and some may be in Philharmonic or Concert pitch (almost a semitone lower than Standard pitch).
This won't matter if you are only going to play the bells on their own but if you are likely to join in with other teams for massed ringing, or use other instruments as well, it may be a problem.
If a lot of restoration is needed, particularly if it involves re-casting and tuning additional or replacement bells, it might be cheaper in the long run to buy new bells. Take English bells along to a bell foundry (e.g. Whitechapel or Taylor's), they will give you a detailed report on each bell and advise you what work (and cost) will be involved.
They will also be able to tell you what the replacement value of the bells would be for insurance purposes.