Well, there is no right answer for that question because it's totally up to you.
My RECOMENDATION is to rush level 400, buy Soultainter and Soulshanks (because of ML bonus) and spend the rest on boosts and preys.
If you don't have a party (4 members), just buy the Soultainter.
I think level 600 is a good level to start buying your full set (including arcanomancer).