I have been to Taplejung a while ago. I trekked along Tamor up to Lungthung (a little north from the confluence of Ghunsha and Tamor). I also went to Pathibhara, Hangpang and also Dovan. As Kalomukh said you won't have problem till the "Sadarmukam". The best time to go there is after rainy season and before winter (October). I read in some other post that you are going to Nepal in October. So good luck. Alternatively you can take a flight from Biratnagar to Suketar airport (above Taplejung bazaar).

Hope that helps

Sparty