When you say change the lock do you mean the cylinder that the key goes in or the whole mechanism?
Euro cylinders are pretty straight forward. There are a few sizes to take into account but they cost between £5-60 for the top of the range. If you were keeping the house I would say go for the more expensive high security ones as the cheap ones are easy to break into.
Locking mechs are pretty complex, There are thousands of different ones and you will need the exact one . Depending on the age and how obsolete it is could cost between £50-200 for the parts.
You need to take the whole strip out and look for names and codes. If you lucky it will have those and a quick Google will find the parts you need. If no markings you need to take dosens of very accurate measurements and search loads of websites.
Gaskets are usually fairly cheap but we could do with seeing the cross section to advise what type you need. Photo of one of the ends head on. Again there are tons of these and once you know the type you will need to find the exact size.
Handles can be a bit tricky to measure too. You need to take these measurements millimeter perfect which can be tricky as you need to measure to the center of holes.
https://www.handlestore.com/blog/how-to-measure-upvc-door-handles/
Personally if your selling I would just leave it to the new owners, Maybe just put a new handle on it to make it look tidy.
You could probably get a new door fitted for £550-650. Which if your planning to do all the above things and you have a tricky/expensive gearbox, One slight mis-measure and doing the repairs could end up costing you more than a new door.
