He has 18k because he has lots of available cash each month.
If he then spends this available cash on running a recent model Porsche 911, along with the £18k, he will lose the £18k and not be able to replenish it at the same rate.
One of these things on its own is probably fine - if he stopped saving but kept his £18k, or if he blew his £18k but was able to replenish it again quite quickly - but both?
And it just seems so uneccesary - he knows so little about Porsche he thought a Boxster costs £12k a year to run. He's obviously not a petrolhead really, he's just a lad who likes a quick car - like most guys - so really is he going to get a proportional benefit to the amount of cash he'll outlay and the amount he'll spend each month?
I doubt it.
If he got something like an Audi S3 with his £18k with a warranty he'd still have a quick car, he'd still have some bling, but he'd crucially maintain his ability to save that gave him the £18k in the first place.
This means he could have fun blowing cash on cool cars without compromising his future
His money, he can do what he wants with it, but as we've no idea who he is we might as well discuss what might be a common situation for many people