Ideally if I get a flat I would want part of the freehold but a lot of them are leasehold, so at how many years does the length of lease suddenly start to make it harder to sell?

at the drop off cafe!!!

Assume you live there until you die at 80, how would it look then?

You have mandatory rights of lease extension under the leasehold reform act. Less than 50 years is usually when mortgagors start asking questions.

If you are in a block that meets certain criteria, you can force an enfranchisement of the freeholdm again under the leasehold reform act.

this site gives you the lowdown.
http://www.lease-advice.org/

Leaseholds arent the problem they once were, just be aware that its worth valuing the lease extension costs on a shorter lease (say less than 75 years) and factor it into your offer price.