Nice looking Luger and a fairly scarce unit mark. Kurassier and other cavalry unit marked Lugers seem to turn up less often than infantry and artillery units. From the photos it looks like "91" is the entire serial number. Very early 1910 production. You have a keeper. You might check to see if the grips match the gun (please remove the left grip very carefully as it is so easy to break off the little section under the safety). If the grips match, I would not be in a hurry to replace them, or if you do, keep the originals with the gun.
A good cleaning is going to improve the appearance quite a bit. A good gun oil and #0000 steel wool will remove a lot of the surface dirt and oxidation without harming the rust blue. Remember to use a light touch with the steel wool. Several minutes polishing in the same spot will damage the blue, even with #0000 steel wool.
I would be very reluctant to shoot this piece. Given that it is all matching, breaking a serial numbered part is not worth the risk. As an Imperial era collector, I think it is a great gun. Congratulations on the inheritance of a nice early military Luger. Also, an excited wife is a boon to any collector!
