For a proper shredder you are looking easily at a few thousand think 5-10k and to have it portable you will need some serious power generation.

Your best bet is to do what i do.

1. Have a professional certificate of destruction.
2. Charge for your service. It's additional work and you lose money when destroying them.
3. Buy a 5 ton electric log splitter. $200-$350 It works great for crushing drives. It's more than enough to destroy them. it will destroy up to 2 at a time. My gas powered can do 7 but i usually just do 5.


Edit.
A paper shredder will not work. Even a wood chipper wont work. you need something designed for metal. Dont try and reinvent the wheel.