The joys of older (in my case, 60's era) homes.

I'm looking for the lowest-cost and most DIY-possible solution to what I'm fearing might be a costly problem.

The metal flange for the toilet (in my ONLY bathroom) is totally deteriorated, to the point where there's simply no place to hook in the flange bolts. The concrete around it is also somewhat deteriorated. The plumber who supposedly "fixed" it the last time (before I realized how bad it really was) simply set another metal flange on top of that old one, but that didn't give the bolts anything to really hold them! So my toilet has apparantly been just sitting there attached to pretty much nothing - no wonder I've gone through 3 wax seals in 6 years!!

We just installed a no-wax seal and so far no leaks, but I'm sure it's only a matter time before it starts leaking again.

My research so far tells me that the "right" way would be to replace the entire elbow pipe connecting the toilet to the main drain with PVC, but I presume that's a very high-cost option.

Is is possible to either
(a) put in some kind of adaptor flange that we can screw into the concrete that will let us continue working right on top of the slab, as we are now? (tile installed directly on concrete) OR
(b) chip off the old busted flange and chip out the damaged concrete, install new concrete there, and screw in a new flange into teh new concrete, OR
(c) install a plywood subfloor to give a new flange something to hold onto and a connector of some kind to make it work, OR
(d) anything else?

Please tell me that there's a solution that won't cost me thousands, as I imagine a totally new toilet pipe would cost!!

is to buy what you call a slip-in flange. Its a flange that has 3 allen set screws that you tighten up against the existing cast-iron pipe. I'm only trying to give you a less cost effective idea like you asked for, obviously leveling and seating the horn of the bowl on the flange is another whole different subject. Again i will reiterate, you can't get this specialty item at a home center, you will need to go to a plumbing supply and talk with a counterman.