Off-road is not about power ... more power just means harder to drive and more frequent crashes!
As Kidderminster (my home club) is fairly small you won't need much motor, a 10.5 will be enough until you are lapping consistently at which point, and only at that point, should you think about going for more power!!
B4 will do fine until you get a bit more practiced as a driver, at that point I strongly recomend going for a Kyosho RB6 (awesomely easy car to drive!)
Battery packs, if LiPo then 2 packs is easily enough to do a race meeting with 2wd. If NiMh then you are best off having a couple extra (but not 100% essential to start with)
Charger will probably just be a matter of getting the correct connectors, easily sorted
"SAFE" tires for Kidderminster are Ballistic buggy green rears and schumacher yellow cut stagger ribs. The tarmac / brick sections are heavy on tire wear so don't be supprised to find you're getting the rears especially quickly. If going to practice I usually setup a layout to miss out the tarmac for this reason.
As for shops I can recomend 3 from personal experience;
JE Models - ask for Nick Caro or Stu Wood, both top guys who really know their stuff great customer service and huge stocks
Inside Line Racing - Kev & Anna are brilliant to deal with and Kev is one of the fastest buggy racers in the UK
Ralls Racing - Nathan will be more than happy to spend time talking you through the ins and outs and again brilliant to deal with, wouldn't be supprised to see Nathan making national A finals a air bit this year!