Yes – tread very carefully!
Firstly the land – it may be reclassified and it may not – you will never fully know and to try to find out can be fun. You need to go and check the local PGOU and use a good local lawyer to see if it may in time be reclassified and what the procedure is. Then take the worse case scenario that it wont be for many many years!!! Re the payment, cash under the table has until recently been common and is still common practice – however it is illegal as both of you are avoiding various tax! If its a spanish owner you may be buying on a compraventa – the same simple form used to buy a car!!! Land or property does not HAVE to be registered in the property registry, only if there is a mortgage involved, so again it could be pie in the sky! If you pay the rest after 50k cash, and get found out then YOU will be liable for the capital gains involved so not recommended in this day and age where the authorites are NOT turning a blind eye to theis
No the use – again until recently people were getting away with placing effectively a caravan on a site, blocking the wheel in but providing there are whells then it is not classified as a property. HOWEVER again the authorities are getting used to this and a couple of owners have had to move/demolish their dwelling!
If I were you I would get a very very good lawyer on your side and talk it through as you may end up severely out of pocket!
Good luck with your choice.