You'll receive a lot of help from the endless sphere forums. I've used endless sphere almost exclusively to solve any problem i've encountered in my ebikes evolution. Your ebike buying experience will be much less painful then mine was as you are willing to seek advice and ask lots of questions. I suspect most people, most men anyway, learn primarily from trial and error, despite being aware that the endless sphere is here to help. Its a pride thing.
I read through this thread, and at some point it seemed as though the 'purchase complete ebike vs purchase parts separately' dilemma had been decided in favour of parts separately? and thats why youre considering various frame options now? Sorry if i've got it wrong, i might have missed something.
I thought i'd chip in on the frame issue. Cromoly will make sure your safe, the metal can bend with the additional force the motor exerts, whereas aluminium can become stressed with the motor forces, and might eventually break. Of course, lots of people can supply anecdotal evidence that 'my ebike is aluminium, and hasnt broken once', but physics is physics, and cromoly is the safety first option. Its a shame that the majority of bikes manufactured these days are aluminium.
If you are thinking of buying the parts separately, then there are several ways to do it. You could buy a frame, then an ebike conversion kit, which includes motor, controller, throttle/ebrakes, and sometimes (though not in the majority of cases) a battery. This route does turn the ebike into a bit of a project, but its totally do able, equivalent to a lot of household diy projects. Perhaps you are thinking of taking the bits off the ebikes you already have, and placing them on a new frame?
whatever the case, balancing weight is one of the first problems to solve. People have recommended placing the motor at the rear to make handling better with the dog trailer, which i would agree with. But it does cause an issue with placement of battery. If yourself/the motor/a rack mounted battery are all located towards the back of the bike, what tends to happen is the front pulls up when accelerating. This is one reason some people opt for front motors; it evens out the weight with the battery at the back and the motor at the front.
If the motor goes at the back, one way to balance it is to place the battery in the middle somewhere, or on a front wheel rack. These options are both diy projects, requiring some creativity.
These considerations can influence frame purchasing. For one thing, some frames dont come with holes to screw a rack into. Without a rack, placement of the battery has to be much more creative.
If you do wish to buy the parts separately, you'll beable to choose the motor you want for the kind of performance you want, and you'll be able to absorb some valuable experience in connecting the parts together, which will allow you to diagose and fix problems yourself.
If you purchase a complete ebike, beware of sales blurb. Most complete ebikes, and alot of ebike kits, are deceptively sold as 'no hassle all rounders'. No hassle all rounders dont exist. Learning why some things work and why some things dont work will help you get exactly what you want and avoid disapointment, and its really great that you are willing to ask questions and understand this subject. A competitive ebike industry isnt fully developed yet, there are few off the shelf solutions, and your approach of trying to understand is definately the best way to approach owning an ebike. The people on endless sphere forum will beable to give you realistic ideas of what different kits and ebikes can actually do in the real world, so ask as many questions as you can, and dont worry about questions being too newbie; knowledge may be an end in itself on other internet forums, and the people there can be judgmental about how much you have, but on endless sphere, i think knowledge is just a means to an end, the end being the ebike grin, the feel of riding. No one will mind where you start, they'll all just want to get you to the grin stage, however long it takes.