1. Maybe not. Depends on what old version, and configuration of the PC you have. They can usually be made to work, but there is that cost in time that may need to be spent. Latest version of RF is only available via download from Steam.
Horizon, who now owns RF, is/was intending to take down their license servers, so it may be problematic getting older registered so it can function. Doesn't impact old installs, but might be a big issue with fresh ones. If so, maybe a call to Horizon can help get it going.
2. Must have the RF box for any version before 8. Besides wear and tear, not all functions for RF 9.5 can be controlled from the dongle. Basic flight controls, yes. But there are several additional handy ones that their dedicated new box does that a TX can't. Your DX8 will be a "dumb" 8 ch transmitter - gets complicated trying to create different models in the TX for each different RF model you want to fly. Better to use one generic, and let the software handle rates, mixing, modes, etc.
3. RF is the most comprehensive around, and a great value for what you pay.
FSOne is a good FREE alternative, just not as full featured. Can accept any USB device as a controller, but with some initial joystick emulation complications.

Both have had issues with non-Nvidia graphics drivers. Both resolved, but it may take some work to find the information as to the right drivers to be installed....