Hi, I can help with a few of your questions. "H3" refers to Gunze Sangyo red paint. Personally, I think plain Testors enamel red is a pretty good match. Are you using acrylic or enamel paint?
If you really want to get picky, the "official" color mixes for those old kits go something like this (all refer to Gunze Sangyo paint):
"Bare White"=95% H1 Gloss White + 1% H16 Yellow-Green + 4% H40 Flat Base
"Bare Red"=96% H3 Gloss Red + 1 or 2 drops of H2 Gloss Black + 4% H40 Flat Base
"Metal Black"=45% H18 Steel + 45% H2 Gloss Black + 10% H40 Flat Base
Hasegawa says to use FS36622 Camouflage Gray for the white areas, BS638 Dark Sea Gray for the inlets and Steel for the feet, and Shine Red for the red. All are available in Tamiya or Gunze Sangyo paint.
As for the water-slide decals, all you really have to do is submerge the decal in hot water for several seconds, let it rest for a few seconds, then use a brush to put it on the kit. If you pre-wet the kit with water or special decal setting solution, you can manipulate the decals a little more before they settle down into place. Be sure you put a sealer onto your kit before you apply decals if you are using flat paint. Otherwise, the decals will "silver" and won't set properly. A lot of people airbrush Future floor wax over the paint for this.
Good luck!
Chad