Armando,
I have enjoyed following your progress so far! The stand build pictures look great (and do I detect a slight modification to the style the KingOfDIY uses for his stands?).
I read through and didn't see any details about a sump - did the previous owner sell you a sump as well? What are the dimensions on it? Like others have pointed out, you might want to consider a removable side panel to install the sump - it might be a tight fit but I'm sure you're handy enough to figure something out.
As far as pumps go, I love (so far) the DC return pump I'm currently using - it's a Current USA DC Return Pump. I've also heard great things about the Reef Octopus and EcoTech DC return pumps. I will be honest here - I've used both AC and DC return pumps. If you're going for silence, DC pumps are the way to go. As far as overflow design is concerned, the Bean-Animal style is the quietest I have experienced. However, that requires 3 pipes and it's hard to tell what the tank you've got is made for.
I also second Pat's recommendation about waterproofing the inside bottom of the stand.
Great progress!