Should I buy a house with asbestos insulation on the pipes?
Asbestos in construction is safe if and only if it is undisturbed, if you move it, cut it, change it in any way this creates dust which is dangerous. Based on what you have said I would not touch this flat unless they can provide a letter from an asbestos removal company stating all asbestos has been removed and no asbestos fibers are present in the flat. Asbestos deaths in the UK are a major health and safety issue as it kills tradesmen every week. Be careful this is your life you are gambling with do not get emotionally attached to this flat.
