The first thing I would keep in mind is that the Pro-Ject won't play 78 speed without buying and installing the 33/78 rpm motor pulley to replace the standard 33/45 rpm pulley. You would lose the ability to play 45 rpm singles without putting the original motor pulley back on.
Also, this table does not have a removable headshell, so swapping cartridges is difficult, requires alignment every time and risks damage to the delicate connections at the tonearm wires / cartridge clips every time. Unless you only wanted to play 78 rpm records with this table, a special cartridge for playing 78s seems like a bad idea. A much better idea would be to use a cartridge that a 78 stylus is available for and just swap the stylus when you want to play 78s. There is a 78 stylus available for Ortofon that comes on the Pro-Ject. If you wish to upgrade the cartridge, I would look for something 78 styli are available for.
Are you really going to be playing 78s? They're pretty noisy and best digitized with a noise filter for listening. Most people don't bother with them these days... unless you're really into rare music from that era... You could digitize 78s with a 45RPM turntable and 78 stylus, using software to change the speed. If you're REALLY planning on getting into 78s, I'd probably look at a direct-drive or other table that could be adjusted to play at the several slightly different speeds 78s were recorded at... something with a removable headshell to enable easy cartridge swapping. Different 78s were recorded with different EQ curves as well, so a phono stage for 78s with different settings for them would be useful too.
The Cambridge 640P is a nice phono stage for the money. You couldn't go too wrong there. But, I've been hearing the new Musical Fidelity V-LPS Phono Stage sounds a little smoother and the price is a bit lower too.
Really, if you're not planning on upgrading from the shelf system or to a moving coil cartridge, the class of phono stage you're looking at may be overkill. I recently bought and sent back several "budget audiophile" solid-state phono stages to see if I could find something I liked better than my Jolida JD-9A. The conclusion I came to is that "budget audiophile" is often a bunch of BS. Nothing I bought was any match for my Jolida and some of the phono stages at $200.00 - $300.00+ sounded worse or no better than a $30.00 Behringer PP400 (which might sound just fine with your shelf system).
Good luck!!! Sounds like a pretty nice system you're putting together. Hope you get lots of enjoyment out of it!!!