The raider idea seems good, go for that next, and if you like coven then do that. most units are pretty decent now, you can mostly play what you want and it won't be uncompetitive. The red greif, especially with reavers, is very strong, you have the right idea with tying them up with that before getting your coven stuff in. Ultimately, buy the models that you like and you'll be fine. The start collecting units certainly aren't "trash", they are actually a good staple for your army. Also, your english is fine.
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