Ok so Orochimaru wants to master all ninjutsu so he goes after the sharingan? Wouldn't it make more sense to try and take out Negato and try and take the Rinningan?
Don't forget they were only in the early 20s at this time, so I would believe Oro could beat Hanzo when he was in his prime, as when he attacked Konoha.He knew he had no chance of beating him. If Hanzo couldn't beat him then Orochimaru definitely couldn't, remember Hanzo fought all 3 Sannin by himself.