In France I had quite thin foam mattresses on all the guest beds and they were so comfortable. I think they were only about 6 cms deep.
I've bought a foam over springs mattress for my house --- too soft, it's going in spare room.
Toppers---don't buy Ikea. Slept on one in Scotland, horrendous back ache.
Why not try some out in a bed shop and get some idea of firmness you need?