What do I need to connect an old RCX 2.0 to a computer and is it worth compared to just buying a new one?
Modern hardware/driver support for the RIS is limited, but it's definitely possible to get it working again.  

The main obstacle is transmitting programs to the Lego RCX brick. The RIS 2.0 came with a USB IR Tower, but the driver does not work on 64-bit Windows. There is also a Serial-port IR Tower available, which works perfectly if you have a 9-pin serial-to-USB adapter.  

Here's what you need to buy (they're all very cheap):  


A serial cable (not necessarily a Lego one, any 9-pin serial cable will do)  
A Lego serial IR tower (can probably find on Ebay as well)  
A USB-to-Serial converter (not necessarily this brand, any basic 9-pin serial to USB should work)  
A 9V battery for the serial IR tower.


With the (included) driver installed, the USB-to-Serial will create a virtual COM port in Windows. If it's relevant to you, you can check the number of the COM port in windows Device Manager.  

On the software side of things, you can code for the RCX using Java, NQC (C-like syntax, but much simpler), or the original Lego graphical programming language. You don't need to install any Lego software if you plan on going with one of the 3rd-party Java or NQC options.  

If you do want the Lego programming software but you don't have the disk, you can download an ISO disk image from this guy's share. The Lego software and its installer ran fine on my machine (64-bit Windows 10), with a small caveat: At the end of the installation process, it attempts to install the USB tower driver. When this happens, you need to go to task manager and KILL the installer process, because when the driver (inevitably) fails the whole installation rolls back. If you're having trouble it might also help to right click on the Setup.exe -> Properties -> Compatibility and run it in compatibility mode for Windows 98.  

Inside the Lego software, you should select the Serial IR Tower when prompted because you want it to use the virtual COM port the adapter provides. You can also set the COM port in the Settings from the main menu screen.  

When you first install the software it will try to force you to do tutorials before you can actually do the "freestyle" programming. But you can double-click any greyed-out option to enable it.

And as always, if you need more help you can always post back here in Bricks SE. Good luck!