Let me address some of your questions:
Please tell me by the time you finish reading this, wether this is a Simple, Medium or Complicated project. -- that depends, you have to be very careful in code design (difficult?), you need a good control algorithm (difficult?) you need to write some simple code (difficult?.
I'll use one master, and it would be a PC. -- Here the trouble start, you need control beyond what the PC can do (you need to turn the port OFF) A solution would be a mastet '51 which controlled the slaves and talked to the PC.
RS-232 and 485 are not protocols, they are interface standards.
Master/slave systems of any efficiency are time critical, I would not be comfortable using VB (VB does not exist for the '51, I assume you are taking about BASIC).
Many variables come into play but I doubt very much that you can make the combination of 9600/current_loop/kilometers work. Current loop is notoriously slow, used to be 10 cps aka 110bps. Long distances is the enemy of transmission speed. RS-232 allow 20kbps for up to 50m. One solution would be to stay away from the optocouplers and use isolated 485, Maxim has a very nice (albeit expensive) chip that does that. With that you can get 100kbps at 4km.
Protocol and such is very well covered in previous postings.
The '51 or '52 will do just fine for this, I have had 4 masters and 48 slaves talking with the 485 protocol.
Unless you are risking life threatening results from possible errors, or your records are very long you do not need CRC, a simple checksum will do just fine.
Do I understand that you want to control 100 micros with one master?. If so, you WILL run into trouble. The better approach is a "network of networks", have "submasters" interface between the master and, say, 16 slaves each. Hotplugging is a separate subject, it adds a lot of complexity and processing, if you can live with "plug and restart" you will be much better off.

"But I'm a rookie in serial communication and serial programming."
I hope you are not a rookie in the rest or you might find this cake too tough to chew.

Are we having fun yet?

Erik Malund