Hi OP,
It's been 10 years since I extended the lease on a leasehold property, so my information may be out of date.
I believe that if the current owner starts the process of extending the lease (not sure of exact terminology), they can pass that right to you as the new owner without you then having to wait a further two years.
I think if the remaining lease is over 80 years, there is some kind of accepted calculation used to determine how much it will cost you, but fewer than 80 years and it's plucking numbers from thin air.
My extension was an absolute nightmare and I almost had a breakdown due to the stress of it all, but my lease was about 74 years and the freeholder was particularly difficult (greedy), so that may not be a typical example. I had to go to a tribunal to fight my corner because the freeholder (a large company) was trying to charge me way over the odds and wouldn't negotiate.
I spent about £20k to add 90 years to my lease, including paying the freeholder's costs.
I wouldn't buy leasehold ever again for this reason, but I understand that it's not really a choice for many people, as it wasn't for me at the time.
There used to be an organisation called the Leasehold Advisory Service that provided free specialist advice, but I'm not sure if it still exists.
Good luck, OP. I hope someone else will be along with a different point of view.