How long is the installation in for? This strikes me as a "dedicated hardware" installation rather than a PC. But if it's only a short run you might get away with it.

Your fixture also has a 30 Channel mode.
http://cdb.s3.amazonaws.com/ItemRelated ... _ver_1.pdf

60 channel mode adds in "fine" channels for rgb on all cells.

Could be useful for certain applications. I would expect a fair amount of latency with that much conversion going on.

Help us out here! What hardware have you got available at the moment?

What's a "letting"?

It would help to have a picture (doesn't matter if it's rough) of your proposed setup.

Your modern graphics card is not directly compatible with your old composite gear. You may get away with a cheap device like this to convert your hdmi output. https://www.amazon.co.uk/gp/aw/d/B01H6OM8OI/ref=mp_s_a_1_1?ie=UTF8&qid=1486228906&sr=8-1&pi=AC_SX236_SY340_QL65&keywords=hdmi...

That's a composite video matrix. If you're stuck using that you'll need to convert one of your outputs from HDMI/VGA (whatever you've got spare on your card) to composite BNC. This is not just a cable conversion. You'll need a scan converter device.

I think you already know the answer! Graphics card with three outputs (best card you can afford!) Output one - User Interface Output two - single screen Output three - triplehead2go for your blend (or better still a Datapath X4). I'm sure other ideas will surface but I don't think this is something ...

Another option/idea would be for the cursor / grid etc to invert the colours of anything it crosses below, that would make it immediately contrasting and visible.

I'd look at capture card options, you can then manage the content externally and load the capture card wherever you want to in the composition. I use this to bring in "tweetwall" type content along with cameras and Powerpoint content.

Cracking response by Joris, I'm bookmarking it for future explanations to bosses/clients.