I still like 4e for it. Especially if you add a little tweaking to 4e. Mainly, make a new ritual for Identify, and then make magic items mysterious again (I do this a lot anyway). Also remember, yes, this is a sandbox, so it's not like there is a time limit on the dungeon, which may mean people blow their dailies just figuring they will rest after every encounter to get them back...but even in a sandbox, monsters refill the sandbox in some fashion. Not the whole video-game respawn, so much, but rather, one group of baddies get eliminated or thinned out, another group of baddies move in to fill up the ecosystem. Kill a few guard patrols, they tend to step up patrols, bigger and badder than before. Kill enough underlings so your a thorn in some under-lord's side, said underlord will send his best or go himself to find out what's the situation!
Another optional rule that helps is the inherent bonus rule. Great for scaling up weapons and giving incentives. Needing to get things Identified will give the PCs a reason to go back to town more often. Add in the Training rules, if not for every level, than for when a major new ability is reached, or at least when a new tier or multi-class feat is chosen.
You can control the tendency for gold overload in such games by adjusting prices in town up and down based on Adventurer economy (IE: when the adventurers are more flush and throwing money around, prices jack up, when they are poorer, and thus customs is scarce, prices reduce....and don't forget, that safe city may want to tax or tariff the goods coming in, and the churches that do the resurrection and remove disease type services expect tithing
4e, thus, with very little modification, works excellently in running a sandbox. Granted, 1e was truly designed around the initil Sandbox adventures. BECMI and OD&D were designed with the Sandbox in mind too, considering those rules systems were still pure (IE: focus the rules on combat, leave the out of combat stuff to the players/GM 99% of the time)....but that's also what 4e goes back too. Combat focused rules, rules-lite out of combat, very adaptable, yet very cinamatic and again embraacing the abstraction that RPGs were known for.