16 and have a Cosworth?
Fair play to you!

Seriously though, Cosworths are only getting rarer and you'll regret it if you sell up.
I'm in a similar boat only 18 and have an E36 in storage at the moment, It's only a 318is but I'v about €3k in it so far. I'm gonna slave away at it and eventually I'll put it on the road.

Keep your Cosworth in storage and build it up to something really special and buy a dirt cheap 1.1 Mk3 Fiesta and slam it on some retro wheels and use that for when you turn 17.