As a former 156 f/l owner,all I can suggest is that if you want maximum comfort with good drivability, go back to factory specs. After about 60.000 stock km I changed to yellow bilstein (not sure about the code, are these the B6?) and 30mm lowering springs (can't remember the brand), but the car was still very comfortable on good roads and with good absorbance and handling on not so good ones. If the stock is so expensive as you say, you can look on eBay for used ones from a user that went for lowered setup. You can find them in pretty good condition sometimes.
The B6 shocks from Bilstein are the yellow ones, yes. The 30 mm lowering springs are usually the Eibach Pro-kit (the black ones).
Mine is fitted with the Sportline springs (the red ones) which lower the car to -50 mm. I am unsure of the shocks, however, but I believe they are not stock, at least at the rear (springs wouldn't have fit on Nivomat).
Strange thing, in the papers from previous owners, it looks like the ordered springs were the Pro-kit, not the Sportline.
The Koni FSDs are not stock, actually. But they seem to have been greatly recommended because they combine the best of both worlds: stiff on quick roads and smooth for "cruising". Whence the price. 600 € is actually cheaper than the prices quoted in previous threads.
I found also a kit Bilstein B4 + Eibach Pro-kit for ~500 €. The OEM springs (which are Bilstein B3, I guess) are not really cheaper than Eibach's.
But I recently read some drawbacks on the FDSs (mostly here actually) which gave me second thoughts. I'm here to find the truth.
Some have also said that FSDs + OEM springs actually make the car HIGHER because the springs supports on FSD are around 1 cm higher than standard.
That's why the more recurrent combo is FSDs shocks and Eibach Pro-kit springs.
there is a chance that the car sits lower because shocks are dead. Two weeks ago I replaced standard Nivomat shocks and springs at the rear with Eibach red springs (official lowering number - 20mm) and normal shocks and car is actually sitting higher now than before. Normally high, not crazy high.
But in my case it was easy to spot bad shocks - they were covered in leaks.
Shocks were inspected first by my mechanics, second by the MOT. None was found.
The car sits low just because of lowered springs.
Are you aware that the Nivomat suspension actually sits quite low at idle and get up only when the car has run after a short distance?
I've seen a 156 with tired Nivomats, it was really, really low at idle (can't slip a hand over the tyre), but normally high when running.
So maybe your Nivomats were not totally out of order.