It took about 3-4 hours. I dunno what year Stang you have, but with the 79-86... You can use the 2 1/16th gauges for fuel level, volts, water temp, oil pressure... You just need to massage the holes a little bigger and the slip tightly in.. As far as the speedo and tach goes... I made a back plate as the 3 3/8ths tach and speedo were too small, but the 5 inch was way to big. The backing plate is wood actually... painted black so you can't see it. There is a lot of wiring involved, I believe 18-20 wires, power and ground for light for each gauge, and the signal wire. It was pretty straight forward.. As long as you're good at wiring.
As far as buying used gauges... I guess you could do that... You just never know if you're getting a working gauge, or how good of a condition... I dunno... I don't like buying used anything. Also, you'd have to find all the same gauges, all electrical, all the same size.. I dunno... I would think with all the shipping costs it would be about the same.
Nick