You can choose one of sophisticated payware aircrafts for your cockpit but you should consider some moments:
You can use such interface as ArdSim with it to control these custom datarefs along with standard ones.
On the other hand, for cockpitbuilders, the panel on the screen is not needed at all, no 2D panel, no 3D panel, it only takes up the external view and, what is more important, takes much processing power. Take note - if you have external instruments, the data can't be sent to them faster than X-Plane's FPS, so having low FPS will affect the external instruments as well.
Realistic and highly detailed 3D model of airplane (exterior view) does not affect the flight dynamics of your plane in any way! Only the 'physical' model of the airframe that has been built in PlaneMaker does!
All you need for your "real" cockpit simulator is just the realistic flight dynamics model of the chosen aircraft, which in general can be done by yourself in PlaneMaker.
This is not so hard to do, having all the necessary characteristics of the real plane on hand, of course. You need to make the "physical" model in PlaneMaker.
Note: you don't need any additional 3D software to make aircraft with realistic aerodynamic characteristics.
First, to get a more-or-less realistic aerodynamics model the 'physical' airframe should represent the actual aircraft's outer structure, including wings and all bodies which interact with the air. For this you need to draw the fuselage and other bodies cross-sections, assign all sizes, surface areas and airfoils, wheel sizes and positions.
Then you need to enter all physical characteristics, such as engines type and power, fuel tanks, weights, instrument limits, etc.
There are plenty of such parameters in PlaneMaker, and the more real characteristics you will add to your model the more authentic aircraft you will get.
Having done this you will get a ready to fly aircraft, that will have flight dynamics as close to real as much of the parameters of this airplane you would adjust correctly in PlaneMaker.
If you want to have some "good-looking" outside airplane model, this can be done separately by creating a 3D model of your plane. Again, this is not necessary, only needed if you want to create screenshots or videos of your flights, or need to have detailed parts of the airplane visible from the cockpit (e.g. for the left or right side view).
As an easier way, you can choose an appropriate free aircraft model or one of those included in X-Plane and correct the characteristics and flight dynamics of this plane in PlaneMaker if you think you need to improve them. Find any available technical data, dimensions and manuals for chosen aircraft and use these to edit all physical data in PlaneMaker.
Then you can remove the 2D/3D Panels (all instruments, textures, some .obj files) because all of this is not needed if you build a physical cockpit with an external instrument panels. Or else you can use the "Forwards with Nothing" view during flight. (Optionally you can remove the panel from forward view but leave the side views).