With a limited budget, and many folks confirming that your expectation of buying one that you will both like.. and last any length of time is out of the question... I recommend that you find the stripped down solid Sealy posturepedic from the reserve line.  736 better coils on the reserve line, and without the expensive foam, will be cheaper.. both of what you want.

It is terrible plowing through Sears.com's abysmal search engine but I found this at sears.

Sealy Posturepedic Theory Firm Queen Mattress Only

It has 736 tempered 14 gauge coils.. good.. should last.

Minimum layers of crap foam to go bad.. on which to lay your latex.  Wore out before finding this with box spring, but hopefully you can get this or something close in budget... and with two x one inch of talalay...

Regarding tests:  I have spent hours lying on the mattress in the store.  Up to 2 hours at a time.  I even thought about asking for permission to sleep over!  Could you talk Macy's into that?

In the many hours of in store vs.. at home sleeping overnight, what felt fine... perfect etc. in the store ALWAYS was too firm by the time I got it home.  May be just me with "need firm for my bad back" in the back of my mind.  The problem is that I need...

If you're still shopping, you could look at Keetsa at 69 Mercer in Soho. They sell an innerspring mattress that uses latex comfort layers instead of P/U foam, as well as several memory foam models. I've never been in but they look like good products on the website, and aren't too outrageously priced.
 

I'm guessing all the foam mattresses will be beyond my budget. ...

Good news. I spoke with Sears and they told me that they are going to treat this as a defective mattress. 

Therefore, whether I do an exchange for a different model or decide to return the mattress for a full refund, I will not be charged any pickup/delivery fees, or restocking fees. Of course, if I pick out a different mattress, I pay (or get credited) the difference in price,  but the original transaction is maintained, so that the 18 months at 0% financing on my purchase stays in effect. It took numerous calls and transfers to speak with a customer service rep who had the authority and responsibility to give me a definitive answer, but now that I have it, I think that's very fair policy from Sears. They also granted me a couple additional...

I am about to call Sear.com again.

If I can exchange without a pickup/delivery fee and/or without a restocking fee, and also get a 30 day return policy on the replacement mattress, it may make sense to select another mattress at Sears and at least channel the funds, which would otherwise be lost, into that.

But, several factors influence this:  foremost is if I can select an appropriate mattress and its pricing at Sears is good. I had put off purchasing this mattress for a very very long time due to my finances. Unfortunately, I do not have a cushion of funds to lose while making wrong decisions.

I’m curious, if a consensus is that interconnected springs perform better for lower back pain, what ‘spring chassis’ are people going with? Is there a make/model that is modestly priced...

Had a cheap (no name) mattress given to me about 5 - 6 years ago.  It is labeled as a firm cushion top.  Decided to upgrad to a king size bed, so I bought a Denver Mattress (the name/model slips my mind) for around $800.  It was only 1 - 2 weeks old before it starting sagging in the middle and I was waking up in the middle of the night with numb hands/arms.  Needless to say, I returned it.

Second mattress:  took the advice of many and went the Costco route.  Chose the Sealy Posturepedic Alanson (cheap, cushion firm).  It was supportive, however, felt a little springy and uncomfortable after 3 - 4 weeks.  Sent...