The castors that came with my BS350S and the mobility kit are rubbish. Within the first year of moving it around on my tiled shop floor, the side brackets on two of the castors bent. I could not find suitable replacements, so I replaced the entire base with a custom 4-drawer base on locking castors.
I used 19mm (3/4-inch) plywood for the base carcass and drawer fronts, 15mm plywood for the drawer sides, and 12mm plywood for the drawer bottoms. The selection of plywood was based solely on what I had available. The locking castors on each corner are rated at 75KG each and the locking lever locks the wheel and prevents the castor from rotating.
The footprint of the new base is larger than the base that came with the saw, so there are no induced problems with instability. With only two of the castors locked, I cannot move the saw using more force than I would reasonably use when resawing large planks. I can eventually make it slide, but it will not tip over. With three castors locked, it will tip over before it slides, but this requires unrealistic effort.