Now sure what you are building it up with, but once the cables and brake lines are ran, you can’t do much to adjust the front end...taking out spacers, switching high/low mount.

you should do the fit BEFORE on a fit bike - then copy the coordinates in the bike shop. Once u cut the steerer and brake lines etc any changes are PITA

I'm assuming fitter will have the front adjusted to go higher or lower.

Don’t. Really don’t. The front end on the p5 is not made to play guess and check. Get the fit then talk to the shop about how to set it up.

The P5 is a pig to do fits on. If the fitter has a fit bike they'll likely give you coordinates and send you back to the shop to get the bike adjusted.
I have done a fit on a P5 with just bars and drivetrain - no brakes and shifters - then it is at least possible to move things around on the bike.
Having the shop finish the build would be a big mistake before you have fit coordinates.

As others posted - do NOT get it built before the fit - assuming P5-6 with Aduro / Magura brakes - as you cannot adjust the front end (remove or add steerer tube spacers) once built without having to cut and rebleed brake lines, which is a real PITA (and your LBS will hate you 2x).

I suggest you get the bike shop to a preliminary build but not install the brakes or shifters or cut the steerer tube.

Then go for your fit and only once that is dialed - you may need a few indoor sessions to perfect it - only then get the shop to do the final build to cut and install brakes / shifters etc.

Also, have the shop notch out the back of the stem spacers. That will make any adjustments down the road a WHOLE lot easier. That will allow you to get one out or add one in without opening the hydraulic brake line. Search it on u tube.
And I agree with everything everyone else has warned about.