I tend to agree with you, a kit (MP7.5, Hyper 7, Savage etc etc etc) is cheaper, especially with the weak dollar at the moment but you won't save a huge amount.
Example (from Towerhobbies)
A Mugen MBX5 less a discount from front page of website and shipping will cost you $624, you'll probably get an exchange rate on a C/card of about $1.75 so thats £356.....but....C&E will charge around £50.. total £406. If you shop around in the uk, you can get one for £500.
It definately won't get through customs so you'll pay the VAT at your door when its delivered. You won't save as much as you think BUT you will save £100 or so. (I've been there and done it). It depends on how patient you can be and whether saving £100 is worth the extra wait.
However an engine is ideal at it may slip through customs and the postage won't kill, I've been VERY tempted to get the 8 port, race Hyper 21 for the same reason as you. As the total cost would be about £100 as you say.
I say go for it and tell CML to poke it.......
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