Truth be told, it depends
If you truly love the shirt than no, it is not a mistake (people pay good money for good stuff)
If you despise the shirt and purely bought it for the class, you did pay $100 for a virtual item worth nothing more than brag rights (IMO) since battle wise in game classes are already OP.
If you really wanted the class then you only made a medium mistake since you may not play this game anymore a few years down the road
All in all whether you made a mistake or not is up to you
With the current class style, Yes it will be OP but not OP enough to beat necro is soloing