Please help me - Hot Wheels present - under £10

[summerscoming]

I'm about to buy a present for a wee boy who my daughter is friend's with (5yr old).

I asked his mum what he liked and she said he is crazy about Hot Wheels.

I know very little about Hot Wheels. What would be a good under £10 Hot Wheel present to buy (up to £13 if worth it)? I assume he must have a track if he is really into it, so do I buy cars, or 'add-ons'?

All suggestions really welcome.