Without wishing to sound unhelpful, asking about the cheapest way to own a plane which is likely to be 30 years old and in most cases roughly treated is a bit of a contradiction
You could be lucky of course and get one which has been very well looked after, hangared, etc, but it's unlikely on the UK scene and the Annuals here seem to run at around £5000 plus for a C150/152.
I would totally avoid a PA38. All with be ~ 30 years old, and every one I have seen (I did much of my PPL in them) was trashed, maintained to an absolutely minimal standard, subject to long term water and occassionally massive leaks into the cockpit and fuel tanks (because the owners don't like spending money to fix the seals) and the bill to bring one to a decent standard will be big.
Your best bet is to form a group of several people you get on with well and buy something better than an old heap. Aircraft ownership is a big enough learning curve as it is