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.
The stove I bought is a Hotpoint and it is 30 inches and the space in the counter is just over 30 inches....maybe 30 1/4 at most. When the home inspector is there later in the week, I can find out what wiring in behind that cover plate. I assume 3 conductor for 220 volts.