Brackets need to be steel really.
The issue with welding rather than bolting is the additional force disc brakes are able to apply. On the subject of which, have you considered what you are going to do to rebalance the braking bias. If left as standard the first time you apply the brakes in anger you are likely to whistle off through the nearest hedge backwards. Even on a 4x4 the rear brakes only contibute about 25% of braking effort, on hatchbacks it can be as little as 10%. MAny rear disk cars jjust don't need them but the market demands them for cosmetic and perceived need reasons.