Is it really a good idea to buy an old house which needs some rework?
It depends entirely on the way in which the house is run down.

Hire a reputable house inspector to thoroughly examine it.  It won't be 100% reliable, but the report will give you a good idea of what is wrong and what is still okay with the house.  For instance you might see things like:


You'll need to replace the furnace within a year or two.
The roof shingles/tiles should be good for another 10 years.
The foundation has recently cracked and part of the structure is sagging.


The first one means you know you'll need to spend some money soon, so consider that amount as part of the purchase price of the house.

The second one means you'll have a big expense down the line, so spread that amount out over time.

The third means that you are going to have leaks, with dampness or flooding, and that the building itself might no longer be structurally sound.  Repairing this might end up being more expensive than tearing it down and rebuilding.

If there are problems of that last type, stay away.

If all the problems found are of the first and second type, with predictable cost, and if all the obvious problems you can see yourself are cosmetic, then if you still think it's worth the adjusted total price, go for it.

There will of course be other problems with severity between these, and there will be seemingly small problems that turn out to be cans of worms, so you'll have to use your own judgement. There will always be risk.  Nothing is certain.

It also makes a huge difference whether you are going to do the repairs yourself or hire someone.  You can buy a lot of pipes and fittings for a few hundred pounds, but hiring a plumbing company to do the upgrades could cost you tens of thousands.