We spent more on the mattress than the frame. Was a sprung one from ikea, however dd didn't really sleepy On it as she was in bed with me from birth to 10 months and then to ease the transition we bought a new foam topper, cut ours up and put in her cot so it was literally the same bed she was in with me before. I wouldn't use a foam one for a young child though as they sink into it so if not mobile I can imagine puts risk of suffocation up (dd was runnining at 10 months so definitely mobile!)
General advice here is to have have a waterproof covered removal liner that seals over entire mattress zipped on so unless you remove the matress and unzip wiggling matress out then a child even removing sheets wouldn't get to the mattress. some suggest anti dustmite sheets under any normal bedding also.
I'm reusing dd mattress given she barely had access to it, it does have cover and I'm removing and sterilising the mattress so I've made myself content it is acceptable for reuse as fulfils the guidelines to make safe if are planning on reusing (besides won't be in cot until older anyway)