A head gasket is something a moderately skilled shadetree mechanic can
do themselves in a couple afternoons with pretty basic tools. If you
have a mechanic do it, expect several hundred bucks. You should check
the head for flatness but unless it was overheated it will probably not
be warped. You only want to machine it if you have to, you can only do
that just so many times before you can't take off any more metal.

If you enjoy doing basic car tinkering a 240 is a great car to have,
they're very easy to work on and you can keep them on the road nearly
forever for very little money since parts are plentiful and cheap but
remember that it's almost 20 years old, it will need work to fix all the
little things that have likely been neglected. If you have to pay
someone else to do the work it could very well nickel and dime you to death.