My thoughts are, firstly that it's going to take you years to render your whole vinyl collection in a high enough resolution to make the project worthwhile! (or is there a real deck at the end of it? ) Just teasing..

Secondly, you could place the motor opposite the arm (i.e. furthest away) so that the torque via the belt acts in a line with the arm bearings ala Pink Triangle (on the late PT1s only)/ Funk Firm - see the stereophile Linn modification review for some pics.

And could you get, have cut some ali/honeycomb sheet instead of the thin steel? Would be much stiffer. Or perhaps just carbon fibre might even be more rigid?