Randy, I find the car alternator/PMA conversion interesting, but...it really does belong in a separate thread.
Starting from scratch is just too expensive and too hard for 99.9% of ebikers who wish for something better
Axel was kind enough to send me a rough copy of his DIY 500W RC motor build-instructions. Trust me, it is EASY (though perhaps time-consuming) and VERY cheap. Scalable to whatever power level is desired. I am anxious to see the results of others efforts from this thread. Based on past postings, these folks will certainly provide a selection of clever solutions (and after all, no single design is perfect for all applications).
a ready made package that will work efficiently from 100 watts to 5 Kw and weigh about 5 kg. Besides 3 hall sensors, and controller a complete motor can be purchased for about $200
Low RPM Permanent-Magnet-Motors are not very efficient. Low efficiency equals shorter range per a given battery pack. A $200 Bafang can be used as a 1-kW non-hub, no need to add the halls plus it weighs a lot less than 5-kg/11-lb. It would provide great range from a small battery at a low amp-draw.
Its not just the motor, 3-kW/4-HP at 24V is 125A, at 48V its 62A...which battery and controller would work?
There may be a use for this in a long-tail cargo bike (low-kV, single-stage chain, 2-kW+/3-HP+) so your competition is the Crystalyte 530X hub series. For the "sweet spot" of 500W-1500W I would recommend the alt/PMA conversion is not even close to being competitive.