Hi gamafn,
I think post #2 here should help 
This would depend on what mattress you buy and what type of foundation is most suitable (see post #2 here). In general though I would tend towards a slatted foundation over a solid surface because it will provide better ventilation (see post #9 here). Plywood over an active boxspring (the kind with springs that flex) can also be unstable and I would treat this as a temporary solution at best and not a long term one. Most manufacturers will sell a suitable foundation with their mattress or recommend the type which works best but there are also alternatives for those who need or want them in post #1 here.
Phoenix