1) How is this currently worth? 50%? 60%? 70%? Is there significant differences between metal and plastic (apart from obvious ones)? What is 'accepted' discount for used, metal PP miniatures?
It's worth about $100 new if you got from an online retailer with a healthy discount like Miniature Market, maybe more like $130 new full retail. $85-$90 is probably a fair price for the models if they're basically in mint condition.
PP Plastics are far easier to assemble than their metal ones, but are a noticeable step down in detail (no game changing but more than you'd see in a
GW plastic vs metal). Metals have marginally more poseability, but they're all pretty static.
2) More importantly: How powerful is this army? I mean I have absolutely no idea what's good and what's bad. I'm not going to participate in any tournament so it doesn't have to be tops but I don't want to get complete garbage. How is that for a foundation for a bigger army?
It's not an army. It's a caster, a loose collection of warjacks,a medicore unit and a solo. I mean you can legally field this as force mind you, it'll just not perform very well at all.
EDIT: Most of the pieces are solid are their own merits, this just doesn't constitute a coherent force. If you stuck to "Mangled Metal" format games (Warjacks only) you could put together something serviceable.