Hi Gaz,
I think i spoke to you at the weekend, with regard to ESC and motor you will probably be best going through
www.insidelineracing.co.uk he does Tekin and Orion which are both top of the range and won't need upgrading, they are also inside your budget. He also carries Durango kits and stock so will be a good place to go for spares etc, if you speak to him he may also be able to do you a good deal if you get everything through him.
He is a local at Stotfold club as well as Silverstone club, and is generally at most meetings around mid south.
I would go for a 7.5 turn motor, no higher. You could get away with an 8.5 and once you are confident with this run some boost on your speedo. The 10.5/17.5 classes are for Touring car. In off-road it is just 2wd and 4wd.
Hope this helps.
John