@Elizabeth Njuguna everything in KC built before 80s and 90s was either cast iron, clay, some combination of those, or on a rare occasion Orangeburg, which they stopped using in the 40s. Cast and clay are very normal. That line could last 100 more years or could go in the next year. Even plastic breaks, it is rare but it happens.
1. Don't be scared or apprehensive because the line is cast and clay. That is very normal. My house has clay. Most of the houses we flip are cast and clay. My personal house has to augered once every couple of years because roots grow into the line where the gaskets connect, that is normal.
2. The seller needs to install a clean-out on the main stack. They don't need to do an outside clean-out. Be very careful what plumber you use. Many big companies will come in and say you need to do a ton of extra work that isn't needed but the seller does need to install a cleanout on the main stack and then have the line cleaned out. Then allow you to inspect. If they don't want to do that you need to either cancel or accept the house as-is and ask for a price reduction or the seller to pay a portion of your closing cost.
3. Are you using an agent? If yes, hopefully, they are good and an investor also. If they are - they should easily be able to handle this for you and protect you.
Hope that helps. Feel free to DM me if you need anything else.