Bertho
Just using a monocrystalline diamond tooling will not give you a superb finish as there are other factors that contribute to achieving optical finishes. The use of fluid film bearings for both the spindle and slideways has a major impact as things like spindle runout and stiction cause major scratches although on a microscopic scale. I have seen diamond tools on ebay however i would not like to say how good they are. I would suggest you firstly buy a PCD (polycrystalline diamond) insert from ebay for about 10 pounds and see if this improves it enough and the next step i would suggest is purchase a cheap air bearing spindle one of which i have picked for less than 50 quid. You make a mount to use it in a standard lathe and this will significantly improve the asynchronous runout which is the biggest factor in surface finish whereas synchronous runout affects form. Hope this helps
Regards
Flannman