Test the brakes and report back (where are you on the graph - good brakes should be around 10% or lower, all 3: front, rear, handbrake).
As for rear axle mounts, here is an example DIY repair. Sandblasting is ideal for such rusty parts.
View attachment 480026
Easiest repair is small welds, spots, just re-attach broken bits.
View attachment 480027
Or reinforce it, make a patch.
View attachment 480028
Sandblasting doesn't help when parts are rusty. Welds are porous (no matter what you do, welding metod - this was TIG). Primer coat.
View attachment 480029
Put some quality, durable paint (coating), 2K (paint + hardener).
View attachment 480030
This should pass the "MOT" in other countries (with more strict rules). If not, you buy and install brand new ones.
View attachment 480031