I'm a 300lb+ guy too, and my suggestion is 2-fold. Consider a fat tire bike, as the wheel pressures won't have to be so high and it will soak up the vibrations and shock loads better, and if you want to ride some trails and gravel, add a strong suspension seat post like the cirrus kinect xl to reduce the shocks to the bike from the top-down. That will massively reduce the peak loads you are placing on it from above and below.
And a few other big guy tips even though you didn't ask: ;-P
1. Don't trust cheap plastic pedals. Aluminum or alloy downhill-style only. Plastic can fail catastrophically, while the metal pedals will destroy the bearing first and give you plenty of warning. Greaseable pedals will last longer with maintenance.
2. Make sure your torque specs are always up to snuff. I have had factory assembled forks and handlebars turn way out of alignment suddenly when I had to panic stop or hit the ditch/curb to avoid a car/hazard. I always verify my torques with a little princess-auto torque wrench.
3. Maintain your tire pressures. Rolling resistance is obvious, but the wheel deforms a lot more when the tire is under-pressure, and you will notice it as the brakes start to rub in corners, spokes flex, etc. Air back up, and it goes away. Check the spokes for relatively even tightness when you check the air as well.
4. You can break chains or bend sprockets with overly aggressive stand up pedaling. Especially when adding e-bike motor strain. Be smooth and steady when up on the pedals.
Most bikes have a 280-300lb weight limit, so you can't let it scare you off. There are some 330lb+ limit bikes like the Biktrix Juggernaut ultra 1000, but to be honest they are all pretty much the same bike in that class, and the weight rating is somewhat arbitrary. I ride a Rize RX Pro with a suntour suspension seat post (I will upgrade it soon - but its holding up fine) and a nice upgraded seat, and it hauls my fat ass wonderfully. While it is rated at 300lbs, and I'm 320, it works fine and I carry a 15-20lb pannier to and from work, or several bags of groceries all the time.
And make sure you do get a mid-drive. Seriously. Just do it. Many builders rate their smaller hub bikes higher weight/cargo capacity since the bike frame and parts are lighter, and that lets them stay within the total load limits set by the hub/axle/crank manufacturers, but it doesn't equate to real performance. I also have a 750w rear hub RX, and it's night and day. They are virtually the same frame and parts but I can overheat and overwhelm the hub at will, while the mid-drive 1000 is absolutely unstoppable by just dropping a couple of gears.
