Go to Orient Photo at 6th floor Sim Lim Square. They are one of the best for accessories.
1. Get a Hoya HMC or Kenko Pro-1 Digital protector filter. No need UV as UV only affects film and not modern day digital cameras.
2. Get a 3rd party hood will do.
3. For screen protector, get a GSS. It is hard and partially hardened glass. When installed, it is super clear and very resistant to scratches. The people in the store will install it nicely for you. Remember to bring your camera there.
4. You need a dry cab. A 30L one will do. If you plan to get more lenses, get a 50L one. You will also need an ok tripod set up if you intend to take lanscape or nightscene pictures. Around 200 will get you a good aluminium setup with ballhead. Just talk to the people at the store. Ok brands are sirui, benro. Manfrotto and Gitzo are a little out of your range now. They are quite quiet people, will respond only when you ask them questions. If not, feel free to browse as long as you want, no one will bother you there.
Hood for the 18-200 would be EW-78D. Best to get one.
Filter just get a normal Hoya HMC UV filter.
I dont think you really need a bigger bag now as it's enough for your current kit.
And flash, you should probably learn more before getting into flash photography.