I really like the models in Island of Blood, most of my skaven are from it.
Skaven have the best half with the most useful stuff, the sculpts are excellent and easy to paint. Island of blood has the only Poisoned wind mortars from
GW which look amazing and the warp fire throwers are also awesome. Rat ogres sculpts are WAY better than the ones you buy separately.
For $40 you can't go wrong as you get the rulebook too! I would go for it.
As for painting skin on the High Elves I can't help you with that

For Skaven it's pretty straight forward in the citadel line this is how I do it.
Prime models white, watered down wash of mournfang brown, then you put ratskin flesh on the skin parts about 2 layers should get good coverage, wash in agrax earthshade or the ogryn flesh one (not sure if its a different name now) then either highlight right away with a brighter ratskin tone using a mix or re apply the ratskin flesh everywhere except the crevices and then highlight with 50/50 ratskin flesh and something lighter (can't remember off the top of my head)
With the clan rats you can easily get away with ratskin flesh then a wash because you'll be painting a ton of them.
I have a rat ogre in my Skaven gallery that I did using this technique and it's from the Island of Blood set.