It is better not to buy S7 if you want to solo. The difficulty is rising fast, it will take you many years to find a block.
Actually with current hashrate estimated on bitwisdom:
Bitcoin Difficulty: 209,453,158,595
Estimated Next Difficulty: 218,702,264,592 (+4.42%)
Adjust time: After 1462 Blocks, About 10.0 days
Hashrate(?): 1,410,294,600 GH/s
This may drop a little after having (we just don't know) but it will over time go up. So you have smaller and smaller chance. You would be better off pool mining with your S7 in my view, as it is guarenteed BTC. If you solo'ed a block yes HUGE reward but chances are very very small with 1 S7. So possible yes... but very unlikely.