Field coil is just one characteristic in designing a driver. Other characteristics are at least if not more important:
motor strength, light cone, low qts,qes, inductance, high efficiency, spider , v.c.,construction quality,
etc...all have an impact on the sound.
Just an example: the fact of removing the surround, will have an improvement in the sound far greater than having a field coil motor.
What driver size do you want? 5-6-7-8-12 inch ,are all possibilities. You should go to different co that makes field coil and ask
for a specefic model that would suit your requirements. A field coil will be practicle if you go open-baffle: you can increase the
voltage and increase the qts for better bass.
To give an example : the Fertin 20ex model 7= 2000euros a piece.
as for the ''regular model with a surround and higher qts=764euros a piece''. The Emspeaker LB12 field coil=920euros.
You have other co that makes field coil: Phy-Supravox-Shindo Latour-atelier rullit(never heard very much about these).