I assume since you're in Germany your standard of a "good-handling car" is higher than here in the States. Instead of wasting your money on a coilover conversion which is unnecessary on these cars except for better header clearance, try these:
-- 1.03" or 1.08" diameter torsion bars (or similar), I like Sway-A-Way myself but there are many vendors for those now
*-- Adjustable strut rods
*-- Delrin lower control arm bushings
-- Box the lower control arms (weld a plate with holes on the bottom to tie the two halves together for strength and rigidity)
-- Front and rear sway bars, Hellwig or Firm Feel are the only brands that fit well and the rear should be adjustable
-- Tubular upper control arms, mainly so you can align it with more caster for modern tires
-- Bilstein RCD shocks/dampers
(Items with the "*" mean they need to be installed together to get the full benefit)
All these parts you can get from
@BergmanAutoCraft , hit up Peter he's a great guy to deal with and super knowledgeable.
For disc brakes look at
DoctorDiff Drivetrain Components he has various levels of kits. The 11.75" Stage 2 kit is probably what I'd go for, I have the 10.95" ones on my '70 Duster I swapped from a later A-body and they're sufficient for hard braking up to about 80-90 MPH. Doctor Diff also sells a rear disc brake conversion kit.