OP, you can get motor data directly from thrustcurve.org
That would probably be more acurate than trying to fit a curve to the motor. Adding noise does nothing to make things more predictable.
Likewise, you will probably want to do interpolation between altitude points for your air density (I assume calculated with an expected temperature and humidity?) at regular points. There's an online calculator for this as well that you might use as a model, can't remember where the website is at the moment.
Another problem that needs tackling is drag profile. If you are shooting for the Karman line, then you will likely have speeds in excess of Mach 4, so you need to take into account Mach effects also (to be accurate) and nose cone profiles/fin shapes that are better suited (good luck finding quantitative data there).
I'm running into many of these same issues designing my flight simulator. As the engineer/programmer/ceo/decision maker, I have to make a decision what can be sacrificed while still providing the end-user with a "good enough" simulation.
As
@UhClem pointed to, your biggest issue is going to involve angle of attack changing the CP and CD of the rocket while it's accelerating. Also oscillations caused by that, fin flutter, and spin. Those are going to eat your motor's lunch for thrust efficiency.
If you want to do it the easy way by putting in some coefficient that jumbles all of that into "noise data," then why ask about accuracy? Just curious, not trying to be snarky though I can see how that would come off that way.
That said, some of the above can be modeled and some can be added to an unknown noise constant (spin is often very unpredictable, buy you can gauge the spin of a "good" build by watching some youtube rocket vids). I'm hoping to model some of the launch effects with my program to a slight degree of accuracy as a failure prevention warning. Wind effects on long rockets can be approximated at slower speeds and they do have an effect on performance.
#EDIT: OR is also open source. You can download their code from GIT (I assume) and look at it yourself to see what they are doing.