Tezpur takes around 4hrs to reach from Guwahati airport. We stayed at the Tezpur University guest house here, which is a few kilometers away from the main Tezpur town, so I can't say much about the main town. I've heard its big and bustling.
You'll find a lot of tour operators who will arrange for vehicles to take you to Tawang from here. You can also apply and get your Inner Line Permit (ILP) from here in case you haven't gotten one made yet. Tezpur lies on the banks of the mighty Brahmaputra river. And yes, it is mighty. To reach Tezpur you cross a bridge built across the river, which is a few kms long !
If you have time, wandering along its sandy banks is sure to be fun. I personally believe if you are going to go to Tawang, you should hire a local driver from there itself not Assam. They tend to know the roads better and are more comfortable driving through bad weather incaase there is any. Also, do insist on a big vehicle like a Tata Sumo or a Bolero.
The journey through Sela Pass and to Bum-la Pass is through steep, winding roads.
A few contacts I collected on my journey:Mr.Wangchuk- 094028592723A local driver who does shared jeep rides from Tezpur to Tawang in a day for you budget travelers. Mr.Mukesh-09436248216/03794-224563A local tour operator in Tawang who can organise for a vehicle for your entire trip. He can also organise local sightseeing around Tawang. Mr. Benu-09401291447/09864182452An Assamese tour operator who does budget Tawang tours. Be warned, these Assamese guys are sulky and do not like being stopped often just to enjoy the journey. I speak from experience.