Meh herb to me is pretty useless, but at the higher levels in P2P I guess you use a lot of pots and brews so it might be worth it, depending on what your RS lifestyle is like.
You basically buy construction levels but it is pretty sweet being able to show off a nice house, I'd say 70 is a good level to be at.
And as Chimp said, make sure you have enough dhide sets and DDS's to survive Jagex