I'd agree with your sentiments. I have Billys on my 155- very good and stable. You just wouldn't know they're there until going round bends- much better stability and body control without ride quality reduction
Konis are ok, but it's relatively obvious an uprated damper is fitted. (Tried a 145QV last month with them). A bit more shock enters the cabin, but also they are noisier by transmitting the bump noises.
The Bilsteins are likely too expensive though. However, you could pick and mix. Rear Bilsteins are priced well, and perhaps Spax adjustables for the front. That leave you free to choose springs.
I believe that Eibach springs are very good (minimal ride quality loss), but if your simply after handling, then I have a set of not much used Activ springs if you're interested. They lower the car about 15-20mm and vastly reduce body roll. There is a fair deterioration in ride quality though, and my wife (and I) found them to be too uncompromising (the 155 is only used for road stuff now). They will fit the 155, 146 or 145. I bought them originally listed for a Lancia Dedra.
If you do decide to go for the Koni kit, I'm sure that you will find it ok. Ride quality and handling will not be inferior to most other combos cheaper than Bilsteins and Eibachs.
PS it's usually a good idea to change the front strut top mounts. You will notice a difference even on an 80k mile car.