My 30 has rear wheels so you can tip it and push it around (although I will be putting it on a cart soon to elevate it and make it easier to use). I use an extension cord but it is a very short one (about 10 feet). It doesn't heat up at all during use. I would be afraid of using too long of a cord or using a cheap one. If you have a high quality heavy gauge cord that is rated to handle the draw of the smoker you should be fine.
As for your other questions, I don't know about the 40 but I can ran my 30 this weekend with a 7 lb pork but and a 6 lb pork loin and had plenty of room to spare so if you plan on doing small batches it should be fine. I am not a fan of extended warranties for anything. I have seen numerous posts on this site on how to fix the few problems that arise on the MES so if you are handy I would say you could repair it yourself and save the money on the warranty.