Would I be wasting my money to get a turntable?

The basic answer is ’energy efficiency’ - by converting the (voltage amplified) signal into a PWM (or other modulation technique) signal, and then filtering that PWM to retrieve the audio signal, the transistors in the final stage are always ’open’ or ’closed’, thus dissipating minimal energy.

Note that in many power amps the analogue-to-digital conversion happens only at the final power stage, which is where ’traditional’ amps consume most energy, however the NAD seems to convert phono signals to digital earlier than that.

Digitizing itself isn’t going to kill the vinyl sound; I’ve had quite good results with Meridian 808i DAC / preamp units that digitize all analog inputs input and apply DAC / filtering on output - even with very high end vinyl gear. It can be suprisingly transparent. The main thing is that if you're used to a tube preamp, you won't get its added warmth and fullness from any digital preamps (as line stages they are a bit clean and dry) - though you COULD pipe the digital preamp into an input of a tube preamp...

That said, I also had the NAD M12 digital preamp and did not like its sound, at all. The M22 amp was fine, but the M12 did not make the cut here - I think even their older M51 DAC might have been better.

My amp gives me the option of converting the signal to digital or keep it analog. Digital conversion sounds better.

One class D is not digital in anyway,

Two his amp has room correction software so if he is using it yes its digitizing all inputs regardless.

Three if you dont' have records already i would pass on them its expemcive to do well, takes up considerable room, takes a long time to accumulate records, new records are expencive. If your a collector type maybe records are for you. 

Would I be wasting my money to get a turntable?

if you hook it up, turn it on, put albums on the platter, drop the needle and it produces sound, no, no you won’t be wasting money. Just don’t skip any of the above...