I also decided to build my own.  IMO the cost asked for the aftermarket bezels, which were almost all just cheap plastic, was absurd.  I modified a repro '67 bezel, built a new rear plate from 1/8" thick aluminum, and installed Autometer gauges.  I can help you with your questions:

1. Modern gauges run on 12V, so the voltage regulator is no longer needed, just a keyed 12V power source.
2. I used a modified stock harness for my gauges, and used the factory light socket wires (ground and power) for the aftermarket gauges.
3. If you're running a voltmeter you just connect it to 12V keyed power.  I removed the wiring for the ammeter all the way back to the factory plug (depinned it from the connector on the gauge harness side).  The only way this might be an issue is if you still run an externally regulated alternator (I converted to a 3G when I did my cluster), because the idiot light provides resistance in the circuit or something necessary for feedback.  I believe, if you stick with the stock system, that you can just substitute a resistor of the correct value in its place though. 
4. Yep.  The factory wiring connects the Autometer sender to the Autometer gauge in my car.  The Autometer sender even had a post type connection, so I literally just plugged the factory connection in and it looks factory (except for the adapter bushing I had to run when converting to the Edelbrock intake).
5. Same deal as the water temp, and the sender also allowed use of unmodified factory wiring.
6. Completely avoid the factory tach wiring diagrams.  They will confuse and confound you because the factory drove the tach like three different ways and none of it works with a modern tach.  I thought I'd be able to just plug and play this one, but nothing I tried worked.  Simply connect a wire from the - side of the coil to the tack signal terminal on the tach.
7. I also connected the indicator lights to the factory wiring, though '67 had two turn signal indicators, so that's a bit of difference, but so long as you know which wires do which t-signal it should be straightforward, and if not they can be tested easily.  On this project I used spring clip mounted bulb sockets in the metal backing plate I built, and I built tunnels from the plate to the bezel sealed on both ends with silicone.  It seems overkill, but if you look at the way the factory did it you understand that without doing that you get light bleed and the colored light comes out other places in the dash.  Another option I've used on other projects are small colored lights that go in from the front in a 5/8" hole.  You can find these at most auto parts stores in the electrical aisle. 

Hope that helps.  Post back or PM me if you have any other questions.  My '67 cluster was a fairly easy project.  I've previously built dashes for my '86 IROC and '87 K5 Blazer (very complicated with angles, AC vents, HVAC controls, trans indicator, and stereo built from a steel frame and aluminum face plates). 

Also, Autometer builds mechanical speedometers that work with the factory speedo cable.  Its a simpler setup that the VSS driven speedos, which in some cases work well (EFI for example if you need a VSS output for cruise or the PCM, or a gearing combination that's impossible to make work with commonly available speedo gears).