Jabz

First time post, so go easy on me I am going to build a shed from scratch on a concrete flag base. If I build the base out of tantalised 4x2 with a blackjack coating on the bottom and sides, would I be ok to make the rest of the shed framework out of untreated 3x2? I plan on treating it afterwards with stain and sealant. Also, what would be the most cost effective method and material for the floor panels?