I was in the same situation the last year.
Have you fought against Marines or better, Wolves, in your games ? It's a good way to check how the different units works.
I recommend this blog : http://space-wolves-grey.blogspot.com/
There are several inputs, from the previous dex and the current, that are useful.
You can read too the 1d4chan tactics article, some infos are useful : http://1d4chan.org/wiki/Warhammer_40,000/Tactics/Space_Wolves%287E%29
They are 2 main way to build efficient armies:
- Thunderwolves and bikes (I love the wolfy theme so I built my army around this: 8 TWC and 16 wolve, termies in drop pods or flyers and static longfangs)
- Drop pods + MSU with Melta (I recommend 3rd pary melta bitz, like kromlech magma rifles because no melta com in the wolves kits, oh, and check their Sons of Thor bits too)
The codex expansion has a very good value if you plan to use wolfguards or thunderwolves too.
Useless units ?
-None is totally useless, but Blood claws and their kin on bikes and jetpack are less interesting, unless you stuff a full load of them in the wolf flyer and drop them in CC.
- Predators, ... all the battle tanks are beaten for firepower by the Longfangs, eventually on rides (razorbacks mostly)
- Wolves are too weak to be a good addition, but they are cheap and fast so they can hunt other weak units.
- Assault Termies without drop pods, land raiders or stormwolf.
As for your other questions:
The battleforce is the "Warriors of the fang" ? then aside the assault unit, usually seen lesser useful in SW armies, you have enough infantry to start a basic army of Grey Hunter/BloodClaws/Wolfguard. You get too a start of Longfangs support. And you get a drop pod if you plan to run your army this way.
No new dex is coming until at least the next year (no rumours). The last is from 2014 IIRC.
A Wolves army can be a good ally for Tau to pack some counterassault units or backfield 1st turn theat on drop pods to let your Tau move forward.