All good thoughts, though note that the Propella has no suspension.
The price and weight are going to be limiting factors. For that price range, even if you went up a bit, figure a hardtail (front suspension only) not front and rear suspension. The front suspension does seem more important to me, but my back is okay, it's my hands and neck that are more problematic.
The good news is, if you went the TSDZ conversion route, you could be talking far lower weight than 60lbs, as low as 42-45 pounds-- but be careful selecting the donor bike, that's where it gets tricky. The lighter donor bikes will have a wider bottom bracket, which will create chainline problems. Mine cost over twice your price because I wanted very low weight (42 pounds), but first gear is unusable, and even second has chain drops unexpectedly. You also have to have the mechanical chops to do the install yourself, or work with a builder. Also, forget CF-- can't really mount a TSDZ, the bottom bracket is even wider, and I don't know if anyone really knows how a CF frame would react to the stress.
There is some controversy about this, but my opinion so far is Tongshens do make you work! A friend of mine has a Bafang conversion in NYC on a cheaper eMTB hardtail, it's probably 47 pounds or so, and he gets a good workout, but it does have thinner tires, and he's finding the ride very hard. I take Gordon's points, but lean towards Indianjo's advice: There are so many variables. I'm a wiry guy, 150 pounds and 6 foot 1, fit but some health problems, and to me, light weight is really important. There are a lot of hills where I live, and some tricky cornering on broken pavement, dirt, gravel and many unpredictable surfaces. I really need at least 2.1 inch tires, which I run a little under their PSI limit, and I know that a lighter bike handles better for my body type.
I do ride my bike unassisted on flat terrain or a very mild uphill grade. I wouldn't buy a bike so heavy that I had to be using the motor all the time, and for me, that's what I would have to do with a bike over 50 pounds.