Only Bangkok ? it's a city only and I think only for Bangkok 15-20k more than enough but excluding flights. but will suggest you to add Phuket. it's a very beautiful place and has an awesome night life . it will take approx take 12 hrs from Bangkok by road but it will be preferable if you can go by flights. you should plan for Phuket and its a worth.
for night life in Bangkok will recommend you to go on Khao San road. bit crowded but definitely you will enjoy cause it's a party hub. you can use Grab app for taxi or Tuk Tuk(negotiable) for budget transport.