Oh dear, sorry to hear about your woes.
Is it definitely exhaust fumes? I would say a manifold leak if you are sure it is, but you would hear it. Could be a crank case breather pipe (nasty oil fumes), or even a rocker gasket.
With the best will in the world, it is very difficult to diagnose on an internet forum - I'd get the car back to the garage and get them to have a look for you.
I'm afraid it is a case of expensive car (when new), and big running costs - no matter the age. This is why so many of these end up on the scrapper
Keep us posted on how you get on
