"And if this sudden jihadist had not been so stupid? How many dead?"
If he'd actually been able to buy a car bomb online (and how exactly was he expecting that to work? The postie was just going to turn up one morning with a bomb in a box? How much more transparently a setup would it have to be?), then he'd have wound up killing himself, and maybe his mum (who I'm assuming is the 45 y/o who was arrested with him).
This dude is clearly not the sharpest tool in the box, I'd guess his next move would have been to declare jihad on facebook using his own account.