I've been reading and reading these forums, what a wonderful group of people here!  I hope some will be willing to give me some advice and knowledge.  I have a 3.5 yr old S bed.  It sags and is extremely uncomfortable.  We put a folded blanket under the sag months ago, bought a (latex I think) topper, and we've again reached the point of not sleeping well.  I'm 5'5 130 lbs (and growing for the next 8 months or so ) normally a side and stomach sleeper, and DP is 6'2 150 lbs side and maybe back sleeper.  I have some back problems.  I'm seen flobeds and sleep like a bear and others.  As much as I agree with the great policies and product, it's just not in my price range.  No bed is actually in my price range and due to several other factors, they won't be months from now either, plus we're hurting too much to wait much longer.

I'm looking at the 7" extra firm M grade foam from overnightmattress as a starting point.  I like their return policy and the price.  But what is M-grade?  What does it mean in terms of firmness?  Would I be better or comparable with the Lux from foamdistributing?

Is the 7" M grade enough for the base?  Is it possibly enough to get a 2 inch latex topper from Foam by Mail then the 1" topper I already have and this might be enough to start?

Last set of questions - can we test this out on the floor or the current box spring, then if it works buy or build our foundation?

I realize this is a very trial and error thing and everyone is different, but I need to start somewhere and I'm getting overwhelmed.  Also considering mattress surgery on our guest bed, which we are assuming has at least a chance of the springs not being shot.  However, DIY projects move VERY slowly in our household, so no telling when that will be done (or even started).

Thanks so much for any advice!