looking for some advise (Bangkok)

Hello from Barcelona!

I will be travelling to Bangkok the 6th of August. My idea is to stay about three/four days in Bangkok and then go to Chiang Mai and Chiang Rai.
Then I would come back to Bangkok and go to Camboya.

I have some doubts about it and maybe you could give some advise:

-What would be a central and reasonable good are to stay in a hostel/hotel. Do you recommend me anyone?

-I would like to travel, the way up to Chiang Mai with the SPRINTER train and the way back by plane. Is a good idea?

-To go to Camboya. Do you recommend me a travel agency in Bangkok that organise “not very touristical” travels to Phnom Phenm-Angkor?

Sorry to aks you a lot ☺
Thank you,
Marta