The question I have, is how difficult, and where would I start,
How difficult depends on how realistic you want it to be and on your DIY skills. Where to start? Read as much as you can on this forum and other cockpit/simpit building forums.
I'd like to drive real gauges if possible, but if thats not feasible, whats the best way to go about a 4th monitor and make it actually look like real gauges and not a screen?
Real gauges are possible using servos and interface boards such as Phidgets and/or OpenCockpits (links in the sticky at the top of this forum), however the 4th monitor may be the cheaper solution using gauge software such as FSXpand (again you'll find the link in the sticky), which is not too expensive, is fully configurable from J3 type gauges to B747 and you can try before you buy. You place the monitor behind your instrument panel, with cutouts for the gauges. If the instrument panel is not too thick and with some neat gauge bezels in front, these can quite look the part.
No doubt John (Darth Vador) Baymore will soon be along to welcome you down this dimly lit way with a few more suggetions.
Regards,
...........Adrian19