New mattress or new bed?

I think I need to replace my mattress.
I have a king sized bed, Harrison Spinks, with a four drawer base.
I have read that ideally a new mattress should not be put on an old base as it can affect the quality. If I am going to replace the base as well as the mattress I don’t need a king sized bed, now I am on my own, but then I’m not sure I will be able to reuse the headboard, which I really like.
I am also undecided how much I need to spend. Yes, the money is there, but I don’t want to spend a small fortune.
I am sure I shouldn’t need to, to get a decent mattress.

So do I need a new base or just a mattress?
Will a cheaper ( not dirt cheap) mattress be good enough? How much is it worth spending?
My spare room has just a narrow single bed which is not very comfortable. But realistically I never get overnight visitors.
Too many decisions. I keep putting them off.