Re prices, yes you can buy a working 4-seater plane for £20k but it will still be a relative wreck.
Again it depends on what 'class' of aeroplane one is going for.
Example - our aeroplane is 'worth' about 25k. It was completely rebuilt in 2005, completely corrosion free, nice paint job, leather interior, 600 hrs on the engine, top condition and well maintained with no expense spared. Being a Simple SEP it is relatively cheap to maintain and run.
It is really
worth 40k to us as an aeroplane but the market doesn't support that and so if we decided to sell it the price has to be right, which is 25k in our case. So for 25k you can get a decent 4 seater, semi-aerobatic, good useful load of 350kg, good STOL performance, great vis, reasonable cruise speed and burn. Not great for IFR I admit as the wing loading is too low and makes for an uncomfortable ride in turbulent IMC but you could shoot an approach with the right avionics if you wanted to. I've flown it around for the past half decade and been all over the place in it and had some great fun. It is also extemely safe to get experience in as it doesn't bite.
I'd certainly take someone with experience along with you when you look at aircraft and even consider getting a survey done to prevent any unexpected surprises!