Carl Zeiss Jena produced a wide range of lenses for the Contax II/III cameras before the second World War. The post-war Contax IIa/IIIa lenses will also fit as the same mount was carried over to the post-war models.
The pre-war lenses were uncoated and whilst of high quality may not be suitable for use with modern colour films. In addition many are expensive collectors items.
The post-war lenses certainly are suitable for modern use as they were coated and in some cases re-computed for more modern high resolution film. Again they are expensive.

A better option for regular use are the Russian copies of pre-war lenses. The Russians took the optical designs and equipment as part of war reparations; the lenses are almost exact copies with some optical re-computation and coating.
They are widely available and generally of very high quality. They were produced at various optical works including Arsenal, KMZ and LZOS.

The same applies to the viewfinders. The best one is the Zeiss Universal Viewfinder, a copy of which was produced by the Russians for use on the Kiev, Zorki and FED rangefinder cameras.
I have listed a range of lenses that are available both as Zeiss originals and Russian copies. The illustrations and text are drawn from the 1939 4th edition of Contax Photography, published by Zeiss Ikon.

Zeiss Biogon f/2.8, focal length 1 3/8th inches, 3.5 cm. Six elements - one front and one back air spaced, two sets of two cemented lenses in the centre.
Russian copy: Jupiter 12.
An entirely new type of objective which combines high rapidity with a wide angle of view. It consists of four components. Those in the front and rear are uncemented and the two in the centre each consist of two elements cemented together.
Definition and evenness of illumination are excellent, even at full aperture - indeed the Biogon represents the greatest advance in the construction of wide angle lenses for miniature cameras that has taken place in recent years

Zeiss Sonnar f/2, focal length 3 3/8th inches, 8.5 cm. Six elements - back two and centre three cemented, front one air spaced.
Russian copy: Jupiter 9.
The increased focal length of this lens, which makes close pictures possible from increased distances, is combined with a very large full aperture. It is consequently very useful for photographing rapid movement under trying light conditions, and for exposure in the theatre, for speed or press work and so on. The definition given by the Sonnar even at full aperture is perfect.

Zeiss Sonnar f/4, focal length 5 3/8th inches, 13.5 cm. Four elements - centre two cemented, one front and one back air spaced.
Russian copy: Jupiter 11.
A special long distance lens for distant landscapes, architectural details, press photography from distant points, studies of timid wild animals etc. The optical qualities and corrections are remarkably good, and the image being sharp and fully covered at full aperture. The short projection of the lens from the camera is particularly convenient.

Universal viewfinder (made by Carl Zeiss Jena)
Russian copies available as original or with turret mirrored to overhang on the left.
Is corrected for five different focal length of lenses. A revolving disc similar to that on the nosepiece of a microscope is fitted with the appropriate optical equipment, and by turning the disc the field of any of the five lenses is automatically show in the finder.

What quality can I expect from these lenses?
Whilst not quite capable of the uncompromising quality available from todays computer designed lenses, the coated post-war Zeiss and Russian versions of these lenses will deliver very satisfying detailed images. Colour correction is generally very good especially with a quality transparency film such as Fuji, or a fine medium speed black and white. It is a good idea to stop down to around f/8 to f/11 if you can as by modern standards performance wide open is not so good - remember that, when new, most other cameras were medium format roll films with 85 mm lenses of maximum f/4.5 aperture!

Compared to modern lenses, these seem positively compact - a small outfit back will contain all these lenses plus two Contax bodies.

If you have a computer and good quality scanner, judicious processing in Paintshop Pro will further enhance image quality.