Stock 1989 318 w/ 48k Miles - Rebuild Recommendations

I agree with the post above regarding the minimal approach. There's two choices. Throw it in with minimal gasket changes, or tear it down and at least do rings and bearings, and do the valve seat refresh you mention. if you want to tank the block, do the refresh - minimal cost for an extra 50,000 miles (thinking 100,000 on a low cost rebuild) . If you leave things be you can probably get at least another 50,000 before you tear it all down and rebuild - If you catch it early you might get away with rings and bearings at that point for another 100,000 miles.