Compromis - what to check?

The notary will check all the necessary official documents and should tell you if he/she thinks anything else should be checked - an example (more appropriate to a house) being that the constructed building conforms with the original permis d'urbanisme.

Apart from that checking is down to things that concern you - how much the charges will really be being a prime example.

But if you have already signed an option without relevant conditions you are committed to purchase anyway, even if your checking throws up a "nasty". That is why you need a notary on board even before you make an offer.

When we bought our place we faxed the compromis to our notaris before we signed it. He checked it and made suggestions for a couple of amendments which were made and that was it. He dealt with everything else.
You pay enough for their service so let them do the donkey work though, as Becasse points out, once you sign there is no backing out whatever turns up.

I can only echo the comments made by the previous two posters - get the notary to check it out, that's what you're paying them for after all :)