Hi and welcome to the forum! We race at West Bridgford in Notts every Friday, and have rd1 of the Indoor Offroad series coming up this Saturday. If you fancy coming down there will be plenty of advice on hand I'm sure. B4's are popular here, they are good for spares availability. That would be my main advice, get a car that is popular in your area then when you break stuff you won't have any issues buying/borrowing/aquiring spares.
As for specs, most down our way run LRP/Novak/Losi Brushless systems. I run Losi Excelorin and orion 3800 lipos. As long as it's a sensored motor/speedo you can't go far wrong. In 2wd I would say an 8.5 would be plenty quick, but most use between 6.5-10.5.
Come down on Saturday! I used to live in Derby it's not far atall. If you come in on the A52 and follow it round South (counter-clockwise)the club is just 3 mins from the last big roundabout with the A60.
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Kawasaki Zg1000