The assessment from the workshop indicates the box is not viable to rebuild, there may be some salvageable parts but the cost effective option seems to be sourcing a used box.
Try talking to John Cockburn (J C Racing) 01845527777 or Nigel Moseley 01952501464. Tell them Will @ Geoff Steel Racing told you to ring them. Dont know if they have one but may be able to point you in the right direction!! (I havent got one!!)
does anyone know rufly what sort of budget you'd need to rebuild an e30 m3 gearbox?
I've got one in my garage with knackard layshaft bearings (broke a number of years ago) and the 2nd hand replacement box is slightly noisy, tempted to have the old one rebuilt so it's ready to go in.