For the purchase of gold, only one character is banned, or the entire account? And are IP/secondary accounts banned for this? I bought gold on my character, and threw a sincere ticket, because I don't want to play dishonestly anymore. I am interested in whether only the character where the gold was bought will be banned, or the entire account and secondary accounts?
That is, if right now, just in case, I create a new account and upgrade it from scratch without interacting with the old ones in any way, can I be sure that this account will not be touched?
You can be sure, unless you break the rules again. Punishments are for "the account", not "the person". Bans on IPs are for heavy and excessive things, for example, if you keep making accounts and use them for bots/rmt, etc.
So play clean and you wont have any problems. Make sure any of that gold/items from banned account does not flow in this account, be it trade/mail or AH, like Obnoxious said.