Southeast Asia forum

Hey fellow travelers,
I'm 24 y.o. female travelling solo. That will be my first exotic solo travel so I'm really excited also a little scared
I'm in a stage of planning and I'm looking for advices.
I'll travel for around 30-40 days and I'll start on 2 Nov. in Bangkok, my final destination is Ha noi. In Thailand I'll visit Bangkok for 4 days (take a day trip to ayutthaya) then I'll go via airplane to Chiang Mai and stay there for 2 days, then I'll take the bus back to Bangkok. I want to visit Koh Samet for a few days. After that I'm heading to Cambodia, I read really bad reviews about the boarder crossing but I think with all the advices on the net I'll manage. My next stop will be Siam Reap for 3 days, bus to Phnom Penh 3 days. I'm thinking of taking the speed boat to Ho chi minh. I know that the boat interesting way to cross the boarder but is it worth it? After that I'll travel with train to Nha Trang, Hue and finaly Ha noi. I want ot visit Sapa and Halong while I'm there.
I think that November-December is good time to visit Thailand and Cambodia, but I'm not sure for north Vietnam? Also the language barrier concerns me, can I buy bus and train tickets in English?
So do you have any suggestion, maybe places that I've missed?
Thanks.