Clarock, a couple of additional comments. Before my Galibier, I used a VPI 19 Mk.III with SDS and VPI JMW-10. Cartridge was a Glider M2. I went to the Galibier Gavia, first with what is now called the Serac platter (PVC) and I then upgraded to the previous top platter, a Teflon/aluminum platter with lead and oil damping. I've used the VPI arm, an AudioQuest PT-6 and a 12" Moerch DP-6.
The Galibier Gavia is a dramatic improvement over the VPI 19 Mk. III in terms of detail, dynamics, background noise (the background becomes black), and speed stability. These comments apply regardless of platter, arm or cartridges that I've tried. I haven't heard any of the more recent VPI tables, but since you're using the same VPI I had perhaps my comments are relevant.
The Benz cartridges work really well with each of my arms, so I don't think you'll have any compatabilty problems there. That applies not only to the Glider but to the L2 that I am using now.
If I were you, I would think in terms of putting more of my money into the table (in order to get something like the Gavia) and less into the arm or cartridge. From my experience, I know I could live quite happily with the Gavia and something like the AQ PT-6 or a similar arm like your Rega 300 and a decent cartridge like the Glider. Of course, it is also easy to upgrade in the future to an even better sounding arm and cartridge, but the point is you could be happy with a setup like that without any need to upgrade.
I would also recommend the Moerch arms. They are genuine over-achievers.
I hope some of this is useful.
Dave