What determines the volume of a Turntable?

Should work fine. Output is combination of cartidge mV and phono amplification sent to preamp.

What determines the volume of a Turntable?

It's tempting to say, "Dunk it in a bathtub like Archimedes", but I won't. ;-)

1. Cartridge output
2. Phono stage gain
3. Line stage gain
4. Amplifier gain
5. Speaker sensitivity

These are the major factors affecting SPL's in your room when playing a record. Of course #s 3-5 affect SPL's when playing any source component.

...how do I know which Phono Pre Amp to buy with my Turntable?

The idea is to match cartridge output and phono stage gain. The goal is to send a signal to your line stage that's similar in level (voltage) to line level sources like your CDP.

KABUSA's phono preamp gain computer is a good resource for calculating suitable phono stage gain for a given cartridge output, or vice-versa. Plug in the parameter you know, in your case the gain of the NAD phono stage, and it will tell you approximately what output level cartridge you need. Understand there's some latitude here, but you don't want to be wildly off.

Also, the recording itself is variable in that some are recorded more softly/loudly than others, and the speed of the record too has an effect. 45's are louder than 33's

yeah everything in your chain on such a tiny signal will alter it somewhat all the way to speaker sensitivity. Also cables and how good they are can effect the energy you get out of this tiny vibrating signal coming all the way from that little needle in the groove... Warning some Albums are just lower output too, so don't expect consistent gain/volumes out of all vinyl just the same as CD has no real standard in output.

First off, you never want to be able to hear your turntable!

But I think what you are asking primarily relates to the combination of phono stage gain (dB) and cartridge output (mV). A search will find calculators for best matches such as the KAB site Dougdeacon suggests. As a general rule and starting point, moving magnet and high output moving coil cartridges with >1.0 mV output may work into a standard built-in phono stage. A medium or low output moving coil cartridge (not as likely a match in your system) with <1.0 will require an additional gain section (active or transformer) either built-in or inserted ahead of your standard phono stage.