These are all good basic questions to ask of your local Rockwell Automation distributor, since they're the ones who will be selling you the software. Every RA distributor has at least one logic controller specialist per region whose whole job is selling and supporting these controllers and software.
There is quite a bit of variety in the SLC-500 and MicroLogix family so there will be some "it depends" answers.
The MicroLogix controllers are considered little brothers to the SLC-500 controllers, and use largely the same instruction set and the same editing software, "RSLogix 500"
There are several "Editions" of RSLogix 500, with different prices and features, to suit different users. The best summary of these is on Rockwell's website:
Because you need to work on both an SLC-500 and MicroLogix controllers, the "RSLogix Micro" is not for you.
You need at least one special cable; 1761-CBL-PM02. This goes from your PC's serial port to the round DIN8 port on the MicroLogix 1200.
You may need other cables depending on the model of SLC-5/0x controller. Some need a USB/DH485 network adapter, some just need an ordinary null-modem serial cable with DB9F connectors on both ends. Rockwell Automation's standard serial cable for the SLC-5/03, 5/04, and 5/05 controllers is the 1747-CP3 or 1756-CP3, both of which have a low-profile right-angle connector to fit inside the controller door. I frequently use generic cables with a null-modem adapter plug on the end.
If your SLC-5/0x controller is a 5/04 or 5/05 model, you might need a DH+ network adapter, or an Ethernet adapter, or you might be able to use the serial port.
RSLinx software provides a communications layer for Rockwell controllers. You need "RSLinx Classic Lite" for use with RSLogix and these controllers, which is free of charge and is included with RSLogix 500.
Versions 5.0 through current (8.40) of RSLogix 500 are compatible with Windows XP Professional. Windows XP Home is specifically not supported by RSLogix.
RSLogix software, like all Rockwell Software products, is protected by an activation system that restricts its installation. Typically a single license is installed on a single computer, but you can get a USB "dongle" hardware device that acts as a hardware identifier to allow you to install the software on multiple PCs and just carry around the "dongle" to activate it on one at a time.
For just one SLC-500 and one MicroLogix controller, I think the best Edition is "RSLogix 500 Starter". While it lacks the online editing features of RSLogix 500 Standard, it is significantly less expensive at around $600 USD.