The biggest advantage to 2WD is that the rear motor can be run fairly low power which makes the throttle smooth and the system intergrates well w/ the PAS, yet, when the frt. motor is engaged the rider can have a powerful bike.
But your requirement don't request for big power, so 2WD is not needed.
Of all the motors you listed, your best bet is the BPM. For 90% of riders, the mid speed is the best compromise, so the 270 rpm rated @ 36 V is what you want. In a 26 " whl. on 48 Volts, top speed would be in the mid 20's mph, a good spot for most bikes. 18 mph is boring and many of us think the low 20's mph is the "sweet spot" for street riding. Having the CST is no big deal, since w/ a BPM on a fair amount of Current, you will end up doing very little shifting, probably 2, maybe 3 gears. Given the big price difference between the CST and the standard free whl fitment motor, I would just get the standard and belly up the price of the DNP Enoch free whl.
Given you would be going w/ a budget motor, take the money you save and buy a BMS Battery Li-Ion Battery w/ the Panasonic cells and the most capacity you can afford. You will save some money by combining the freight.
Unless your time is worth nothing, I don't see fiddle farting w/ tool batteries as being a "good idea". Read thru the projects here, I don't think any of them turned out that great. There are weight and bms issues.
The last item to complete the power system is the controller and here you need to research and decide if you want to go w' the new crop of sine wave controllers and their displays packed w/ features and functions. Or go w/ a simple square wave controller that will not need programing. If you keep the controller under 40 Amps, the 3-speed speed limited PAS will work fairly well, but the 5-level CURRENT limited sine wave equipped units will offer better feel.
i want to be able to pedal without resistance (or very little ) when not using the motor
You can forget that. Some serious cyclists using a very sm. batt. pack and a mini-motor report being able the ride almost normal w/ the power off, but w/ around an added 30 lb.s and a slight amount of added resistance, you will not. What we do is select the speed/power level of the PAS to corraspond to the amount of power you feel like adding w/ the legs. Feeling frisky that day, select a low setting and pedal like Hell. Feeling lazy, a higher level and less leg input. But what I'm saying is, the power stays on all the time.
You have not given us any info on the donor bike, but what ever it is, it will probably need some improvements or modification(Ebikes have a way of begging for mods,tires at the very least), so don't ignore it when it comes to determining your budget.
My recommendation to you, start reading up on the types of controllers. The sine waves from PSW Power are populatr right now, but I like the square wave from their sister store, elifebike.
The one down-side to using the BPM w/ a sine wave from BMS Battery is they don't really offer the right size for that motor, which would be a 9-FET w/ a rating between 25 to 40 Amps. What they offer is either too sm. or too big.
The PSW Power controllers only go up to 20 Amps I believe, but they can be easily shunt-modd'et to 25 Amps. Still, a little low, but they would be smooth as silk.