Until someone can categorically determine whether any image presented to them was shot with a DSLR, mirrorless, or for that matter on film, arguing about image quality between the systems is a moot point.
Much more important are the aesthetic qualities of a photo, such as composition, exposure, colour etc, rather than the technical aspects of the tools used.
Theree is no real difference in image quality between the two types, since this depends only on the sensor.
The difference is in the handling. Mirrorless cameras are smaller/lighter, and people can prefer the optical viewfinder of the DSLR to an electronic viewfinder.
Finding a good mirrorless camera for 600 pounds is going to be challenging, while they are ...
DSLR cameras have a mirror that allows for through-the-lens composition with an optical viewfinder. There is no difference in image quality between DSLR and mirrorless cameras that use the same sensor and processing pipeline with the same lens, lighting equipment, and settings.
The use of different sensors, processing, lenses, lighting, settings, etc can ...
Basically I would keep it dry (as suggested already) and prevent it from direct sunlight.
Taking out classical batteries may also be a good idea; maybe even rechargeable ones to avoid deep discharge.
For cameras with a mechanical shutter (the old non-electronic ones) you might also want to release the shutter to relieve the spring moving the shutter.