PC connects to 25 RS-485 Gadgets (like Hubs) and each Gadget inturn connects to approx. 20 Terminals (roughly around 500 in all).

Guess this would facilitate the data transfers I require

Well with no numbers given, I can only say "this will work if the PC can push enough data".

When I last did PC to many 485 devices I made a PC board that had a 485 port for each "submaster".

If you can feed and drain all 25 "RS-485 Gadgets (like Hubs): at 115k through one line you will, of course, only need a B&B 232 to 485 converter for your PC.

Erik