Like a lot of people here I am thinking of buying a firm inner spring mattress and putting my own latex topper on it. I've been looking at various S brands but I wonder if any of them will last. For example, a base Sealy (Bryant Park firm) has only 1/2" "Sealyfoam" and a "supersoft" pressure relief inlay layer. Not much to go wrong there, unless the foam is really cheap - which I fear it might be. Other Sealys use "marvelux" foam, which they say is very firm and higher quality. Does anyone know if either of these foams will last?

Also, how much foam can I get in the mattress before I'm asking for trouble? The base model I mention above is rated as a 2.5 for comfort on US-Mattresses.com - like sleeping on the floor. Obviously 2-3" of soft talalay latex on top will soften that up, but I like a soft  mattress and I'm not sure if it will be enough, and I don't wanna end up stacking 6" of latex on top of a 12" mattress. Some of their "plush" mattresses have like 8" of foam inside! Obviously I wanna avoid that trap but is 2-3" in the mattress reasonable?

Thanks!