Good luck, and enjoy!
Be very cautious with financing at the moment this is the biggest unknown as banks change guidelines weekly. Rates also changing could help or hut your DTI also. Best bet stay in touch and dont take anything for granted.
Yes, you must be pre-aproved. Don't worry about how long a pre-approval letter is good for. I work with many different lenders and once my client is approved for a certain amount I have the lender provide a pre-approval letter for the price and the specific property my client is interested in.
Hope this helps!