Shelley was a key member of a close circle of visionary poets and writers that included Lord Byron; Leigh Hunt; Thomas Love Peacock; and his own second wife, Mary Shelley, the author of Frankenstein.
If the 900 watts means the input power, then 1000 w inverter would be ok. If the microwave produces 900 w of cooking power, you will need a bit more, maybe 1200-1400 w if you don't know what the input power rating of the microwave is.
You will need to drill 2 holes through the top cabinet near the front and in-line with the holes at the top of your microwave. There are 2 bolts with 1 washer each that will go through the cabinet from the top and screw into the top of the microwave. These bolts are usually about 3 inches long. These are the part that actually supports the microwave. The back panel only helps you mount the microwave.