The Lipo you have now is fine, tbh it sounds like you have most of what you need and a different motor will get you the speed on the straight that you are after.
The springs won't break the bank and if no one has some spare on here you need:
Silver rear - AS6478 - £2.69
blue front - AS6497 - £3.89
(don't know why the shorter springs cost more!)
I found Bradders setup from Southport on the CML website, may be worth comparing that with your setup (although he was running green front springs)
http://www.cmldistribution.co.uk/pro...0Southport.pdf