Obviously it is best to get the clocks to work with the lightest weight to reduce the wear, so I would normally start with a heavy weight of about 6 lbs and then gradually back off until it stops working, at which point I would add back the last amount taken off and assume that that will continue to work. I find it easier to use a 2 Litre Coke bottle filled with water initially and then pour out small amounts until until it stops working.
Really you can make the weight from what ever you like, ideally it should complement the aesthetic of the clock and not look bizarre or incongruous. I favour the brass weight shown on the clock images here but I have used a granite block in the past on one of mine.
It is often difficult to achieve the weight that you need in a size envelop that works with the overall design. One solution to this is to construct the weight in two parts using an outer sleeve filled with lead shot. This is a really handy method for achieving the look that you want. You can find suppliers of lead shot on the Internet, this is one example LeadShot.
None of the clocks are what you would call simple because of the complex form of the gears and the accuracy needed to keep the meshing together without sticking. Clock one is the most popular download, and is a straight forward construction, but Clock 11 is probably the simplest. Clock 19 has the most instructions and has been designed to be constructed using just the CNC machine and hand tools. See my article here What do I need to Build a wooden clock
If you subcontract the cutting of the gears to either a laser or Waterjet contractor then 70% of the difficulty is removed, you still get the pleasure of making the rest of the parts and the assembling them, and then debugging it to get it running smoothly and consistently.
You can make the gears from brass and as I have seen from clocks built in this way they really look great. You can use Perspex as well , either clear or coloured to give you a special effect.
The only material I personally do not like to see used is plywood, use MDF if you really must, to quickly machine the parts to see what it will turn out like , but for a clock that you are going to display to friends and family to show of your skill, please try to avoid it.
You can scale the clock as much as you like but without changing pendulum length, as the swing of pendulum is fundamental to the accuracy of the clock. By scaling the pendulum up it will slow the clock down so you need to change the gearing to compensate. You can go to this site to look at the effect of pendulum timing that happens when changing the length. Pendulum calculations
Yes you can as long as I get some credit in any publication of your design.
I have always shied away from any finish that is likely to swell the wood, so I have always finished by spraying on a thin layer of clear polyurethane. After that spray a thin coat of Dry Film Lubricant.
No this is not possible, you can only make a clock for yourself or a family member, you can not make the clocks and sell them for profit.