I've now bought 3 different ebikes, so I though I'd share my (entirely subjective) thoughts on the kind of stuff that works - and doesn't - for ebikes. Here goes...

• Frame: strictly dual-suspension. I don't see why one wouldn't get a dual-sus bike considering with ebikes, it really doesn't matter. As for frame material, don't worry about carbon, just get steel: nice and heavy. Carbon makes no sense for an ebike unless it's a foldable.
• Motor: Bosch or Yamaha are the only ones I know, they are industry-grade motors that you know will work well. As for homemade kits such as Bafang, I will say one thing: torque sensor. This thing makes all the difference, it defines the difference between an ebike and a moped that's completely insensitive to how much effort you put in. Let me just say, the level of enjoyment is very different, and so is the level of safety.
• Batteries: Depends on the range you need. I have Bosch DualBattery (2x500Wh, ~200km range) and I cannot live without it. Traveling on a single battery is annoying, I keep worrying that I will run out of juice. A home-made build will let you make a much larger battery, of course, but with commercial systems Bosch DualBattery is great.
• Rear wheel: Forget derailleurs! They are meaningless on ebikes. Why go for a clunky, unreliable derailleur when you can install a Rohloff 500/14 that will probably live for 10 years? Honestly, now that I've started using geared hubs, I cannot go back, and I keep telling anyone to throw the derailleur away as soon as they get an ebike and replace it. And no, before you ask, there's no issue with high speeds, at least not with Rohloff.
• Wheel size: Most ebikes are 27.5 and you'll have to live with it. If I had to choose, though, I'd go for 24/3.0 or maybe even smaller. Why? Well, to be honest, sooner or later you're going to want to go 50kph and this is when you realize that, oops, your 27.5" bike doesn't give you enough stability. Sadly, this is something that's out of our hands since there are no large 24" frames (though Haibike makes a 24" kids ebike, in case you care).
• Tyre width: I think getting anything less than 2.6 on an ebike is pointless. I would say 3.0 is ideal. Electric fatbikes are awesome, so if you absolutely must do your own Bafang build, get a fatbike! Again, the reason I recommend this is, with an ebike, there are no weight concerns, it's all about comfort. And electric fatbikes are still fairly unique!
• Tubes: the thickest available; for example, if you go with Maxxis, you'd go for Maxxis Downhill (1.5mm thick) on a 24" or 26" bike, Maxxis Freeride on a 27.5" and Maxxis Fat if your tires are thicker than 2.5".
• Tyre type: Well you probably don't want Nobby Nics trust me, I tried, the amount of vibration that you get at 20kph is really annoying, it travels into your backside and your arms. Rock Razors or Table Tops are just fine. If you envisage traveling on roads only, then you want the Hookworm tyre, though personally I lean towards Holy Roller. And yes, this is a 'dirt' tyre with a checkered pattern, very nice.
• Seat, pedals, handlebars: I always get the same set: Serfas Rx Cruiser, Shimano MX80 Saints, NC-17 Trekking. Again, these items are motivated by comfort, which is something you can afford on an ebike since you're not chasing weight economy.
• Lights: I go for Supernova M99 Pro, not exactly cheap, but it automatically adjusts to ambient light (so you can keep it on all the time) and has the low beam/high beam switch, which is super-useful in unlit parks and whatnot. This is a very expensive light, mind you! Also, it's only for Bosch, Yamaha PW cannot handle the 12V power load. For Yamaha I recommend the Lupine SL S, it's half as powerful (only 6V) but also has automatic light adjustment and also a more smooth distribution of light compared to the Supernova.
• Mudguards: Install them. There's no bike where the mudguards don't fit, though as you go with fatter tyres, it becomes more and more tricky.
• Rack: Get one if you can, as large as you can. Again, weight is not an issue on an ebike, so you can bring more food to the picnic or more spare parts for the bike.