I don't think you can delete the stock app. You want to go into menu->settings->accounts and find your gmail account and uncheck sync gmail. The gmail app will still be there, but it won't be getting email anymore.
As far as K9, you have 2 ways, push or poll. For poll, go into K9 account settings and set folder poll check frequency to however often you want it to check.
The better way to do it is as push. Again in account settings, set folders to check with push to "only 1st class folders". Then make sure folder poll check frequency is never.
Push would be only applicable to imap accounts, I think, so set up your servers as IMAP.
This way, it'll automatically and nearly immediately deliver mail to you and still not be bad on the battery.