There's also the Rega Fono Mini, which to my untrained ears has a wider soundstage and than the NAD PP2 and the basic Pro-ject phono amp. I've not heard the 640p. I'm inclined to think the Slee would be a touch of overkill for the Debut, but, if you decide to stick with vinyl, is more future proof.
As to cartridges; the new Ortofon 2M red gets good write-ups on this forum and looks well priced. If you can find an OM10 at a good price, it's a nice, neutral cartridge. The Audio Technica range, starting from the AT110 up, are also good buys if you like a livelier sound.