We just went through this a few weeks ago. Are you using a convertible crib? If so, check out the rails ahead of time and see how high the mattress will be off the floor with box spring. Our mattress is sitting kinda high for our little guy, he can get down no problem but he won't be able to get in bed himself for awhile. They sell low profile box springs that might be worth it.
Mattress salesman are like used car salesman. Have a price you are willing to pay and they will "magically" get there. We went for a model a step or two above the absolute lowest quality. No toddler needs a pillowtop mattress. So something basic but decent. Our mattress and box spring for a double were $699 on sale for $599 with a $50 rebate to get you to $550. We said we weren't paying more than $400. Suddenly they had 1 left of "last years" model. Normally they don't do free shipping for that cost, but of course because we were local they would do it, blah, blah, blah. A manager will have to get involved to "approve" the deal more than likely. If you start to walk out they might stop you and offer a sweeter deal.