Just did a quick check from FAST LANE here and for an 88 300CE a CA cat (Miller) lists for 1,225.
Same item for the other 49 states = $637.

Are you sure it's bad? Although it's 18 years old, 100,000 seems a short life for a CAT. Any noticeable flow restriction? Does your indy think it's the CAT?

And, yes, it is easy to change the odometer, or replace it altogether. They are known to break. Mine went last summer and it was fixed (replaced) by an indy shop.
Are you sure the mileage is correct? For an 88 that's average annual driving of 5,500 miles. Does the condition of the car jive with that little useage??

My 88 CE now has 85,500 on it, and I hope to get more than 15,000 out of the CAT, even though I will need to replace some if not all of the still original exhaust system ( mufflers/pipes) in the near term.

It's been said here a lot:
There's nothing more expensive than a cheap Mercedes!
Good luck with the car... IF you decide to buy it.