Being able to afford a good mattress

Hi there
We have been gifted a superking wooden base (Ercol Bosco, Superking) from a relative who no longer wants it. It is a beautiful base and we will be given it with no mattress.
We don't have a huge disposable income but want to get a decent mattress. They all seem so expensive but we don't want to buy a cheap mattress and regret it. What are your thoughts, what would you recommend?
Kind regards