@ SirGraham
I don't have experience with the IV24, but it sounds like you can just plug a monitor in there and you'll be set. Remember that most modern LCDs don't display 50Hz signals and that PAL video is 50Hz. The Dell monitor described above *is* 50Hz capable. But also note that unless you're using the framegrabbing capabilities of the IV24, it's sort of redundant as a display card.
Now, the Cybervision. If you've got a monitor attached to the IV24, you're going to have a second monitor (or a switchbox) attached to the Cybervision. More accurately, you should have the monitor attached to the scandoubler. The "Light my fire" screen you're seeing is probably a CyberGraphX bootlogo. CyberGraphX is, essentially, the driver package and API for the Cybervision card. Normally, the bootlogo should disappear when the Workbench appears, but I suspect the previous owner was running this system with a monitor attached to the scandoubler AND a monitor attached to the Cybervision. That means that some screens (such as the bootlogo) aren't closing automatically.
Look for a program called CyberGraphX. It's a configuration tool to set some environment variables for the CyberGraphX driver system. It's usually found in SYS:Tools or SYS:Tools/Commodities.
On the Environment tab, make sure "NOPASSTHROUGH" is disabled. On the Tooltypes tab, make sure "PASSTHROUGH" is enabled. This means that native, scandoubled Amiga video will come through that port *and* video from the Cybervision will be passed through. It allows you to use a single monitor to display Amiga and RTG video, dependent only on which screen is frontmost. You'll need to reboot once these settings are saved.
The next issue is the garbled display you're seeing. I agree with zipper, the configuration of your Cybervision's screenmodes is probably not correct for whatever monitor you're using. The tool for fixing this is CGXMode, normally found in SYS:Prefs. Once you get that open, I suggest picking one of the default sets from the Settings menu. Those are "safe" presets that should work with almost any monitor capable of the refresh rate indicated (use 31KHz to be absolutely safe).
But before you do that, I suggest opening SYS:Prefs/Screenmode. What mode is currently selected? You may want to pick a native mode, such as NTSC (make sure to "Use" and not "Save" for this test), to verify that the scandoubler is working.