Re: how quick would your ad appear on google front page after payment?
It actually depends upon what type of payment method you have used and how early your ad gets approved. Most of the times your ads are approved automatically if there are no policy issues. But if your ad requires some manual review, then it might take a business day or so to get your ads approved.
Read more about ad approval here: https://support.google.com/adwords/answer/1722120?hl=en