No, you've misunderstood how a PLL demodulator works. The NCO (or VCO in an analog system) must track the instantaneous frequency of the incoming signal, not just its average value. If it does this accurately, then then the control value is an accurate replica of the original modulating signal.
If the oscillator only tracks the average frequency, then you have a PM (phase modulation) demodulator, which is related, but different. With wideband FM (such as used for FM broadcast), the instantaneouis phase error will be much greater than 360° at times, and the output of your phase detector will "wrap", createing a lot of distortion.
Having got the datasheet, I'll now give a more general answer as to how I'd approach it:
Firstly, do you need a precise frequency in the first place? There is a certain amount of slack in most applications and you can probably just use one of the internal oscillators. I see this part provides a 32khz RTC oscillator and a 48MHz (USB2) one.
Secondly, is there something that does need a precise frequency that I can use as an input? For example, PAL video or USB. If I have to use a crystal, I don't want to have to add another crystal to the BOM if possible.
If I've got a free choice, pick a crystal in the middle of the operating range of the PLL which is cheap.
Smaller divider values are better for jitter, but check your fractional PLL's available fractions.
For your specific application of a UART, it's common to oversample because there may be jitter on the other end. So you might want a sampling frequency x3 or more times line speed.
It seems to me the accepted answer (by Sparky256) views the PLL simply as a filter and completely ignores its actual purpose, which is being a control system, controlling the phase of a signal. The order of a control system signifies its number of internal states. In a system that has a single input, the states beyond the first state (order) are equivalent to derivatives of the controlled variable.
Specifically, in a PLL, the controlled variable is normally the phase of the signal. The PLL attempts to produce a phase lock. So, the first order is for the phase variable/state, the second state is a derivative of the first state - which is frequency, and so on.
For a simple frequency synthesizer a first order PLL might suffice, but with a QPSK demodulator a first order PLL would probably be lacking as any carrier frequency offset between the modulator and the demodulator will always produce a constant phase lag, which can only be removed by a second order PLL. A phase lag means the I and Q channels cannot be fixed (they constantly "move"). Therefore, a QPSK demodulator should have a PLL with at least 2 states (i.e. 2nd order or higher).
Also, contrary to some notion prevalent in comments and answers here, a higher order doesn't make a system slower, nor does it make it faster. Response time is determined by all of the system parameters, mainly by the value of its coefficients (or the position of its poles and zeros, in filter design jargon).