1. A 50 scope is nice for the added sunlight and details, but you are not on the battlefield to capture some butterflies so I'd say a 40 is a nice balance between sight image and scope size
2. Magnifications over 9x are going to be probably avoided while shooting airsoft guns..you are going to reduce your picture too much and you can live well and with less tunnel vision when set on 9x, which is already a lot. I'd say most of the time I keep it set at 4x and all is good. Screwing up with your zoom while playing in a dynamic environment adds more variables to your perception of mil dots VS effective range..so I'd say a 3-9 is more than enough
3. Mil dot, I kinda like it. Either that or some nice of stepped reference in the crosshairs is going to help you with quick compensations in windage/elevation
4. Once zeroed, you will probably NEVER change your elevation and windage in airsoft rifles. The reason is simple: On a rifle scope, a change of 1 MOA (which translates to something like 1, 2 or 4 clicks on your scope, depending on the model) is equal to a compensation of (approximately) 1 inch at 100 yards (300 feet). In real steel guns, 100 yards is the distance at which you will probably zero in your scope. In airsoft, 100 yards is probably the maximum distance at which you can engage..and probably not even effectively. So why would you care fixing 1 inch or so at 100 yards? Compensating using the knobs is just going to drive you crazy and spend more time twisting those knobs rather than taking a shot.
5. A level inside the scope is going to be a nice addition but it will only work if you leveled your sights and gun properly. The error of cant has to be eliminated in your gun lab before you actually field the weapon. Anyway, this a more complex subject. Make sure your gun is leveled with your sights so you don't lean it left or right while shooting..this will screw your shot up. If you did your homework, a level in the scope is going to be a nice addition..but only if you did