redrum - On leaving it outside in the rain, I would say no because of the electronic PID that is mounted on it. It is dust and water resistant, but it is electronic. When not in use I recommend keeping it covered. SI sells the covers, sometimes they are a bonus or freebie.. If using in the rain, be sure to cap the female connector for the food probe plug. You can also rig up a clear acrylic or plastic box to cover the controller. But if you visit the SI site you will find many folks using their smoker in inclement weather with no issues.
I did a full packer (17 lbs) for Easter lunch. 225 temperature with a target internal temp of 195. Started it just before 4 PM Saturday and pulled it around 11:30 Sunday, wrapped in two layers of foil and into a cooler to rest. We ate at 1 PM and that brisket was juicy and tender. Not once did I worry about whether it would come out good or not. And I did not need to check on it. I relied solely on the electronics and was not disappointed.
Cleaning? I line mine with aluminum foil on the bottom/floor and also the roof of the wood box. It helps tremendously with grease drippings removal. Because the racks are stainless steel, I use scrubby pads to clean. If drippings are pretty much fused to the racks after a very long smoke (think 17 hour full packer brisket smoke) then I use a pumice stone, GENTLY, for the really hard stuck on tidbits. Works very well every time.
There are some folks using the AMNPS with their unit with varying degrees of success. I say varying because the SI smokers are really tight. Most of us use chunks of wood. No need to reload throughout the smoke. So you would have to experiment with the AMNPS. But check the SI site for more information.
I cannot recommend buying another MES as I have zero experience with them. But I have read enough negative posts to rule one out for me. (And yes, I know there are success stories as well). I do have experience with an offset wood smoker that I used for years and still have, as well as a Cookshack electric smoker and the SI Model 3D. Aside from the shipping costs, I truly believe that SI provides the most robust unit with the most features and the best warranty for the money.
Good luck whichever way you decide to go. Dave