I was in Turkey

Off to Turkey for 3 weeks, white water rafting in september. What are the temperatures like at this time?

Travelling around....going to Ezruzum for 2 weeks and back to Istanbul in 5 days. Is travelling around easy, what about long haul bus service?

any info will be appreciated

White water rafting is good in september. I presume you are going to KopruluKanyon in Antalya.
Travelling around is easy. There are so many bus companies. You can book a day or two earlier. Going to Erzurum is long way from istanbul or antalya. However night buses are better to spend time sleeping.

Have fun in Turkey

prepare yourself for cold weather especially around erzurum. travelling with buses is easy. you can find buses almost anywhere anytime. but do not take bus between antalya - erzurum or istanbul - erzurum. both will take around 24 hours and it is not fun after couple of hours. flights are reasonable especially if you book early (almost cheap as bus trip).
you will probably visit yusufeli, go to the restaurant right by the river (i dont remember the name) and drink lots of raki. eat "cag kebab" in the morning (guaranteed heart attack but delicious)