Get a cap or at least use plastic saran type wrap on the 240s master cylinder if there is not a secure closure now. The brake fluid will extract or absorb moisture from the enviroment if it has a way to reach it.

There are still 240ds out there with really good engines to be had pretty cheap. A really good engine rebuild where nothing is ignored can be pretty expensive.

You can remove check and repair common wear items if required on a really good used engine. Actually pretty cheap in comparison. Takes a little hunting one down though.

If you are really serious in restoring the car to tip top condition then another alternative is a metric motors rebuilt but it will cost you. Most these used engines will require machine shop work for a proper rebuild.