What beats a Pentacon 5.6/500 mm with 2x convertor?

[Eugen Mezei]

I intend to get 1000 mm non-mirrot onto my Kiev 88 and to M42.
Now I thought buying a Pentacon 5,6/500 mm and adapt it to Kiev 88 mount. (Should not be difficult.) Unfortunately it seems this lens has some optical faults, especially compared to modern(er) lenses. Ofcourse (I think) it would be better than a 8/500 Wundertüte. (That does not cover 6x6 anyway.)

What alternatives could you recommend me?
It should cover 6x6. Luminosity would be nice but not so important. This will be for tripod takings of buildings far away. Optical quality should be better than of the Pentacon. It should be easily avaible. (I know a russian apo exists, but it is rare.) Price is a factor beyond some limit.