1) Will other vampires attack me?
The hostile ones will still attack you, as if you're a non-vampire. See the UESP wiki article on Vampires.
3) Is it difficult to feed?
Not really. Just feed on sleeping NPCs. You can even feed on your own followers (order them to sleep in a bed and feed on them before they stand up again). See more NPCs that can be easily fed upon at the UESP wiki's "Vampirism - Feeding" article.
Also, a tip from the same article:
The most effective and safe way to feed is merely using the Illusion spell "Invisibility" (the "Quiet Casting" Illusion perk would help too). While using Invisibility there are a few actions which do not interrupt and cancel the spell; feeding is one of these actions. Merely cast it and you are free to feed on any sleeping NPC without being seen, as long as the spell doesn't end in the middle of your feed.
For the rest of your questions, see:
What are the advantages and disadvantages of becoming a vampire?