Resin Casting Question(s)

Watching the progress of the Coach 64 build and other's projects, I realized that resin casting just might solve one of my scratchbuilding dilemas--the need for a number of archbar trucks.  Yes, I could buy them but what's the fun in that, eh?

The trucks will be loosely based on a Bachmann truck but with some of my own designs changes.  No undercuts in the prototype but there will be a few round parts.  I'm expecting to have to build a two-part mold.

But after reading various posts and watching countless videos on YouTube, I have some questions.  Hopefully someone can point me in the right direction.

1) There are a boat-load of products out there in terms of types of resin as well as mold making materials.  How did YOU decide on what you are using?  What characteristics should I look for in the resin and the mold-making?

2) Seems the best material for what I'm going to be casting, in terms of a mold will be urethane.  How does it hold up to mutiple parts/uses?  If not urethane, then silicone?  If silicone, what hardness?

3) I know that most plastic parts these days are injected.  what resin(s) approach the hardness characteristcs of injected plastic without being too brittle?

4) How do you decide how much resin to mix?  Use water and then measure the volume of water? 

Thanks in advance.

Bill W