Is it against forum rules to buy bitcointalk account? I don't want to buy one and then have it deleted by admins.
No. The forum does not support it and they don't ban it either. It is just frowned upon by majority of the members.
I would use escrow of course.
Is it profitable? I want to calculate how much a lets say legendary account is and then see how long I would take to earn it back by joining signature campaigns.
It depends on how many constructive posts can you contribute since all the signature campaign requires quite some post to get paid. The quality of the account matters a lot, many campaigns are not willing to accept accounts that has bad posting history.
Oh. And I wouldn't mind the account being flagged as sold. Actually, I would do it myself. I would not want to see myself in a mess like BitcoinExpert is at the moment with the declouds scam.
Sure. Just be reminded that a bought account is usually an indication that the person have other accounts. Since it is bought, the quality of the previous posts would not be indicative of the future post quality.
I would strongly recommend you to create your own accounts and participate in the forum actively. That way, you can learn more about Bitcoins and possibly earn some money when you are able to participate in signature campaign.