I am using a pro-ject 6.9 with an Ortofon 540 MkII and quite happy with it.I used the 510 too and it also sounded very nice too. I think 520,530 MkII will be better and satisfy your musical needs for a long time.
I had bought a Pro-ject K-6 cartridge at the time I purchased my pro-ject t.table. The Pro-ject cartridges,K-4 and K-6 were made by Grado,I dont know if company still provide these cartridges. I had a problem with the alignment of K-6 .Using the pro-ject cartridge alignment tool ,I understood that the needle of the cart. was a few mm.s frwrd and the exact overhang point could never be aligned . I asked the dealer to give me another K-6 or 4 cart. to try and the result was the same. I could never had the stylus rest on the correct overhang point with these cartrdges. I tried and used some more cartrdges on my pro-ject t.table ,such as Shure V15xMr , Ortofon Super OM 30, Akai PC180 ,Akai PC 200(MC) and none of these cartrdiges caused any alignment ,especially overhang!! problem.
May be these will help you a bit