Catalytic Converter/Check Engine Light

Recently purchased 97 540i. 130K, excellent shape, very clean. Air bag light was on and had an exhaust rattle beneath drivers seat. The dealer had both taken care of right after purchase. I passed the emissions test then, about a week later the "check engine" light comes on. I go to the same BMW/Mercedes Mechanic who then reset the light and suggested it was "probably" the catalytic converter. He said if the light came back on to call, he would order the part and arrange for installation.

My question...How complicated is it to change the catalytic converter?
Where could I order one and get a good deal?
Think it is worth it to purchase one of the engine
light reset tools?

I have the same problem with my 99 528i
replaced them with an afeter market brand and it did not work
dealership says it must be a bmw mfg converter

has anyone had luck with an after market brand?