How long is a piece of string? No one knows.
The M104 engine is tough as nails as long as the head gasket is intact, and I've NEVER heard of a bottom end breaking unless the engine was really abused. Top ends last at least 200K-miles, maybe farther.
The tranny will probably give out before the engine, but most 722.3 and 722.4 units seem to last at least 200K-miles. Again, service history plays an important role.
Keep in mind that a 1996 car with 125K-miles is worth LOTS less than you paid for the car. Depreciation is probably your biggest expense so far. Don't sell it at this point, or you'll be getting the short end of the stick. Some smart cookie will buy the car for next to nothing and then drive it for the next 100K-miles, laughing all the way. I know. I did this with several 190E's.
1998 C230 "Black Betty"