You're going to have a hard time finding a field M/S rig option cheaper than the MKH 30/40 pair...which are less expensive than a Schoeps pair. I have some of the AKG Blue Line bodies (SE300B) with cardioid and hypercardioid capsules, but I've never tried out the figure-8 capsule. I do know that it is noisier than the capsules that I do have. While in no way terrible, the cardioids and hypercardioids do have a noticeable noise floor. I wouldn't suggest them for quiet environments...meaning, I wouldn't suggest an M/S pair of this line for quiet situations/sounds either.

If you're going to be working indoors, I'd say there's nothing to keep you from using some large diaphragm condensers instead of smaller pencil mics. A pair of Audio Technica AT-4050's would produce some nice results (gotta love selectable patterns).

It's not necessary to use microphones from the same manufacturer, but be sure to audition any pairing before laying out the cash. Also, keep in mind that there are other stereo recording techniques that might suit your needs just as well: ORTF, spaced omnis, etc.