Personaly I would get an alloy radio tray (£8 to £20) as this improves the brakes, and possibly a stronger brake servo(£50) or front brakes (£50+). the faster you can stop, the faster you can go in teh first place.
If you are happy with the steering response then just keep the servo you have until it eventualy dies.
Definately geta kill switch (£25+) on it, no point upgrading it then loosing it in a runaway
the stock motor mounts are fine if you are not jumping and arn't making too much power from the engine, so there's potential to save upgrading thease for later. I've not done mine yet but it is on my wish list.
Some shock sox will save you time and money in the long run, and cost from £5 upwards (unless you make your own from some stretchy fabric or 1.75" cycle inner tube).
Additional air filter (outerwears or redkneck (£10)) Pull start cover (£5+) and crank case mods (£2) are a must to save time/money rebuilding the engine.
I got some alloy rear upper arms off ebay for £15, and they massively improve the fore/aft stiffness of the suspension, so i'd highly recommend them (stops the main gear getting munched by the suslension arms in a crash, and sharpens the handling)
I'm going to get some baloon springs for the rear of mine to improve the handling, which are about £10.
From my personal experience, there are reasonable pipes for £40 (see Nimrf models) which will give a good power increase. However I broke my cheapo pipe in a big crash and replaced it with a Jetpro (£120 ish), and can say that the Jetpro gives me twice the improvement that the cheepo pipe gave (I did a thread somewhere with actual accelerations measured using my GPS for stock, cheepo pipe and Jetpro). I would recommend that if getting a pipe, you save up for a DDM, Jetpro or similar as they are worth the extra money.