IMO, if the big-ends are gone, you need a full rebuild. Age is what kills big-ends and there is no point putting new big-end bearings in without sorting the whole engine. I'd get the head sorted too if I were you.

You're probably looking to pay £1500-£2000 for a rebuilt engine I would have thought. Or look out on here in the for sale section, they sometimes come up rebuilt and brand new.