In time you're going to want probably 1 or 2 more boxes of Goblins.
But you shouldn't need to right away. Start with the ones you have, and the new Heroes you're going to get. And get a Drum if you can afford it! 
Those Trolls can really help give you some supporting punch but be very careful about letting them loose on their own...they probably won't last long ( H2K models in WotR can go down in one turn pretty easy).
So that's a good start, but probably a bit weak.
To help your life out a bit with regard to the painting and toughening the force, you could always get a decent point-sink, such as a Dragon or Balrog. It would give you an attractive center to your force, help soak up some points so you don't go insane painting Goblins, and be a good addition for play purposes as well.
For Wargs, you can pick up two boxes of the plastics and have a decent force ( 6 Companies ) at a reasonable price. Use the Beastcaller if you want or just deploy them as normal.
For Spiders I have found that 2-Company Formations are pretty fragile but much more starts to get costly. So you're going to find these an expensive addition if you try to do it with official models and all at once. On the plus side, you can find plastic toy spiders of almost the same size at many toy shops (around here I just check in "buckets of bugs" that go for 0.25 each). Just base coat them and paint them up as you would anything else. They're often not very detailed and would absolutely NOT be allowed in an official game, but it would let you get them on the table cheap and you can start swapping the Companies out one at a time as you can afford the expensive GW blisters.
There are variations such as the Very Hard To Kill ( VH2K ) and Extremely Hard To Kill ( XH2K ) which give some staying power, sometimes several turns on average.