Hi Glen,
the Shark is a good starting point as it's probably the most competitive car at national level (until the Carisma GT14B becomes available). The long wheelbase chassis is not an essential upgrade but it does make the car more stable at speed at the slight expense of not turning as quickly, especially on tighter circuits. At nationals the car is now classed as 1/16th scale to give some of the less race orientated cars a chance in 1/18th scale.
Electrics wise, I use a Novak Mongoose esc with Novak 10.5 turn motor. This is plenty quick enough and small enough to fir comfortably within the car. Gearing wise, you would want to fit a larger spur (49t) and a pinion of around 10/11t to stop it getting too hot.
Servo wise, I use a low profile futaba but many people use micro servos. A standard 1/10th servo will fit but you have to remove the mounting ears.
Take a look at the forum on
www.uk-microrc.co.uk as there's a section dedicated to the Shark. Try rcmodel imports for the chassis kit and top deck/body/shock towers made by Frogger.
Good luck, they're great fun. Hope to see you at some of the national rounds
