The break problem is most likely not too tricky, I would guess the Main cylinder is leaking/Faulty. Its not a big job to replace, it sits in the Engine bay where the Fluid-reservoar is. It needs dismantling and repair, then the breaks needs ”Bleeding” after that.
Here is the main cylinder for about EUR 70 from Sweden: https://shop.speedparts.se/sv/prod/99-90/broms/huvudcylinder/523917.html
They also have the repair kit for a whooping EUR 4.50, if you only want to replace the internals of the cylinder.
On a workshop I dont know how much it would cost, but its a standard job and nothing special at all.
The Steering rack could be more of a trouble. If its leaking its most likely a seal/Gasket somewhere. Due to its position, not entirely easy to fix. It it needs coming out, I think the engine of hte car has to come out. Im not 100% sure on that, but its anyhow a challenge to repair or dismantle with the engine in place.
However, it should be verified where the leakage is coming from? Is it something else leaking maybe? Note that the break cylinder is straight above the Steering column and if that leaks actually the drippings could maybe be found on the steering column. So first of all, locate the real leakage. Maybe its not too bad and an easy fix? In my experience, leakages are sometimes not coming from where you first think.
Leaking steering columns is not very common I would say, at least not up here in the north, even some similar old cars with lots of milage dont usually have problems with this.
Hope you find this useful! Its a nice car the old 900 and getting really rare all over Europe now, especially here in the North where they seem to have rusted away now!
/Martin, owner of a nice Classic 900 1991.