Have the fuel hoses ever been changed? By very careful, that would not be the first 750 to have the engine room burning. Early models even had a recall.
http://www.bimmerboard.com/members/l...re%20Hoses.pdf
I changed the ones on my 750's all, some were hard like stone and I could easily break them by hand, although they are just made of flexible reinforced rubber.
As for the check engine light, do a stomp test and see what signals come out. It is described on my website below.
In any case you have to solve the gas smell problem first immy.