The floor between apartments is insulated with 6 inch fiberglass and one layer of 5/8 drywall.Does anyone have any suggestion on sound buffering between floors?

Considerations I've found so far include: various layers of outdated insulation (mid-80's fiberglass stuff laid over old vermiculite), 1x2's nailed together to create 2x4's (but still relatively sound structure for the age), asbestos siding (needs a powerwashing and I've found it's totally OK, until it's time to rip it off or demoed), 1" galvanized metal water main is very old, electrical wiring that is visible is a hodge podge mess, there's an old gas pump on the site (not sure if tanks are still buried), real bad mold in bathroom, at least 3 layers to the foundation (that looks solid still), ugly old wallpaper, cigarette smoke smell, kitchen is a complete gut and remodel, nearly the whole home is.  

The shower pan is an old worn out fiberglass job. 

My estimates include modest landscaping refreshes and often $10,000 in plumbing/electric, properly refinishing older tubs and sinks, not replacing with fiberglass, proper renovation of wood siding, not covering in vinyl, etc. 

Thus, exterior foam insulation is the best method with regular fiberglass insulation from the inside.

Fiberglass is ok, and concrete is good.  

For around $500, pick up a new craftsman style fiberglass front door (HD has some in stock) and get rid of the screen door.  

On a complete  bathroom remodel... is there any reason I should not use Osb behind the fiberglass  shower stall?

I like fiberglass over vinyl any day of the week. 

We used a fiberglass-strand additive mixed into the paint that was supposed to add strength and life to the shingles.