Once the hammer goes down you have exchanged contracts.
You then normally have 28days (sometimes more) to complete.
Most buyers at an auction would not be paying for a survey beforehand, but that is because most aren’t normal buyers.
It is definitely worth speaking with the auctioneer beforehand, they can sell stuff before the auction. Sometimes they might let you know what the interest level is like.
Based on experience I would suggest that you work out what your limit is and bid aggressively right up to it. If you are timid then anybody bidding against you might “find” an extra bit of money thinking they can knock you out of the running. If you appear to want it regardless then sometimes people drop out.
I know of person who lost out on a house they wanted and the winner was somebody who bought it just because it was cheap.