Don't just go by "convertible" on the box. Most cribs I've seen in physical retailers are advertised on the box as convertible. However, this usually just means that the frame is sturdy enough to stand with one of the long sides removed, and has holes to attach a full-size bed frame. It does not mean that the box includes a guard rail for a toddler bed. It does not mean you don't have to perform quite a bit of disassembly to remove the extra side.
Since most cribs take the standard mattress size, and have completely vertical sides, you're going to lose about the same amount of square footage no matter which crib you go with.
You can find cribs with under-mattress storage drawers, but it's probably just as easy to get separate storage "drawers" to put underneath, if you so need.
Cribs, and many other infant-used products, are regulated by the US Federal Government. This means that there are standards of quality and features that must be met for them to be sold in the USA. Even if you're not in the US, the major infant brands probably still meet the US standards (since the business isn't likely making a non-regulation version for less-dominant markets).
Many crib features, such as adjustable height, are also standard. For many styles, all it takes for the manufacture to make the mattress height adjustable is to drill an extra set of holes (or a few extra sets). It costs them almost nothing to add, but not doing so makes their product much less desirable.
Mattress-related goods, in general, are highly standardized. That's why you can get a standard crib that can convert to a headboard for a full-size mattress. (Crib mattress is 51 5/8 inches "long", full-size mattress is 53 inches wide. This is not a coincidence.) So, most retail cribs will be compatible with various add-ons or accessories, even if from different brands.
We purchased my son's crib at Walmart, on *clearance**, and have had no issues with it specifically. The only problem is that most cribs are too large to fit through standard house doors while fully assembled. Since my son has switched rooms a few times, and we've moved a couple times, I've had to take it apart and reassemble it in the new room many times. Nonetheless, this Graco-brand crib from the world's largest retailer has had no physical or hardware issues and is still just as sturdy as the first assembly. It was a great purchase.