I have some experience with this. Work in real estate in a neighborhood that is predominately late 50s early 60s slab foundation ranch style homes with original cast iron plumbing. That would be my first question, which I didn't see above, are your pipes cast iron or did someone previously re-plumb it to PVC?
In addition to a scope, you can do a hydrostatic test. Plumbers in my area charge about $250. At the exterior cleanout you block the pipe and fill up a tub usually and remove a toilet. Pressurize the system and if the water level holds, no leaks, or you can see it recede slowly, or just not hold at all. They have become so common that the Texas Real Estate Commission now included it in the latest update to the standard residential contract. You have to now get permission from the seller to perform this test during your inspection (as there is a chance that by performing the test you can cause damage). They've become so common that it's basically part of the home inspection, or one of the first questions people ask "Is it original cast iron plumbing, or PVC?"
My only advice if it is cast iron, is don't repair a section. Tunnel under the house and replace all the plumbing with PVC. This can run $15k-$25k depending on the house. If it's PVC and isolated, yes you can cut concrete and repair a section.
Good luck.