Results from the Contax, whatever model, will be of no visual difference to the Nikon. You would be buying exclusive Contax optics which cost a lot more than Nikon optics and for all practical purposes perform no better - they just cost more!
The Titanium Nikon F3 has the same internal workings as the standard F3 and the F3 HP so you would be gaining nothing except a wallet and a camera that weigh a little less. However the Nikon F3 is one of the most reliable cameras ever made and would not be put off buying another if I had need to..
Some zoom AF Nikon lenses are, shall I say not the best, but most are good and will deliver the goods. A lot of unsharp images are down to camera shake/poor focusing (non AF lenses) so the camera lens can hardly be blamed. You don't say what lenses you found unsharp but personally in 40+ years of using Nikon, I have never found any of the non-zoom lenses to be unsharp. The first version of the 80-200 was not the best but was sorted out with the later AIS version.
Buying a camera on E Bay is a gamble! Some sellers are as honest as they day is long, some are out and out rogues only out to fleece you of your hard earned cash. I would NEVER EVER buy a camera from that place unless I could see it and look at it first. A dealer will sell you a camera that will have some sort of warranty and the better ones will not knowingly sell you junk!
Bear in mind that manual and some AF cameras are running short of spares. The Nikon F3 will be no different, but I would hazzard a guess that the spares for Contax models which were produced in smaller numbers will be very difficult to find. The parent company went out of business several years ago. Stick with Nikon you won't go far wrong