Hi,
Turn of century ....so I suppose all cast iron. Look for cracks or repair welds in the base, arm reaching from table to top wheel and around the top wheel frame and case. If cracked then leave it alone as a repair is difficult and potentially expensive. Once cracked then all sorts of alignment diffulties become apparent.
Co-planar refered to the top and bottom wheels being in the same plane. Essential so that they can acccurately track and run a blade a 90 degrees to the table. Best way to check is to use a 6ft level, open both doors, remove table if necessary and lay level on both wheels....bubbles in level will show any misalignment. Then look for the adjustment points on top wheel and see if they are free...or look easy to free up.
Check guides....probably just wooden blocks or metal triangles...if wooden blocks then you can easily replace if metal triangles then you will need to judge if they can be cleaned up or flattened and reused, Not a big job but a bit time consuming. Have a look at top guides thrust arrangements also...often they were just a hard metal rod that could be adjusted back and to to fit blade size.
Turn of century probaly had no dust collection.
Try to see if there are any seized bearings...but you should be able to get new ones cheaply from modern bearing suppliers.....you might have an issue with pressing them out and back in but look at any car repair manual to see ways to do this.
Do you know what make it is and or a model number. Is it working now and being used recently.
regards
Alan