Frankly, cogging is more noticeable in 1/10 than 1/8. When I ran sensorless - I could never tell there was cogging - the Castle firmware for the ESC is really good. The RX8 is a little smoother off the line and when doing 'fine throttle management'.
I wouldn't run the MMP in a truggy - it's only a 1/10th ESC afterall.
4S vs 5S, for the same power (capacity times voltage) will weigh nearly the same (give or take a few grams, nothing more) - so there's no advantage there.
Import duty is always a pain when ordering from abroad. Mike (RC-Monster owner) does usually mark up packages at half-value, but I will say, prepare to add 17.5% of the value of the goods + £8-13.50 on top of your budget for anything coming from the US. This is why it's best to get much of your stuff from the same place - even if another shop is $20 cheaper, by the time you've paid for shipping and import duty, it's not worth it.
Use
www.rclipos.com for good quality LiPos - they do free worldwide shipping, and mark the value of the goods down low. Use RC-Monster if you want their conversion kit for the ESC and motor too.
Pinions, get the HPI ones for the Savage Flux or the Hot Bodies ones for the Ve8 - they aren't too pricy to be honest.
My recommendations:
Budget set-up:
Castle Creations MMM with 2200KV motor
Hyperion VX 4S 5500mAh pack (x2). This should last 15 minute finals, and for longer, just do a battery change.
RC-Monster XT8 motor mount and battery box.
Better set-up:
Tekin RX8 with 1350KV motor. With the Hotwire to set it up on the PC.
ThunderPower G4 30C or 45C 5000mAh 6S LiPo - this should last 20 minutes.
RC-Monster XT8 conversion kit.
RC-Monster Slipperential.
You'll find that when racing longer finals, they turn a little bit more into Le Mans style racing, where you're trying to drive efficiently to get as long as possible out of a battery - it adds a little extra challenge to racing, which is remarkably enjoyable.