Welcome to the board! :)
It's always great to see a fellow World Eater fan.
As far as the plastics go the current version is mostly MkVII. The latest pre-heresy armour would have been the MkIV but you can actually buy the torsos and helmets from Forge World.
You also have a single plastic MkIV helmet and torso on the Black Templars upgrade sprue.
The beauty of the MkIV is that it was notiously difficult to maintain as the techs were just learning to manufacture it as the Heresy broke out. This means that you can afford some variation in it's design as you can say that they have adapted the design due to short supply of materials, etc.
A few other handy suggestions involve avoiding the use of the torsos with the eagle chest emblems and the pointy nosed MkVI helmets.
It's worth checking out the Space Marine painting guide like
Thocan said.
The main thing about Pre-Heresy that attracts alot of gamers is the conversion aspect. They provide you with a wealth of conversion opportunity and because the background material is fairly scarse it allows you to be creative.
BL artwork books like you say are probably the best visual reference for conversions and inspirration.
Here's a couple of examples of my own WE that may give you some ideas.
Astarte

Although I've extended the legs here this model is made using the Forge World Red Scorpion Veterans MkIV parts.
The bolter is a second ed 40k plastic and the leg plates are from the terminator sprues.
The key I think when making pre-heresy armies is the inspiration. For 40k I generally think along the lines of gothic/medieval version of Dune, everything is very ornate and highly decorated. Suits are often relics and treated with reverance.
For 30k armies I think of the clone armies from star wars, everything is mass manufactured and functional. Suits are there to provide protection and parts can be replaced.
Pre-Heresy Kharn.

The Assault marine torso with the eagles filed off can make an excellent example of battlefield modification of MkIV armour. The exposed power cables have been enclosed to give them extra protection.