I'd look for around 1000w, mainly because that seems to be both powerful enough to avoid the nuisance slowness issues you mentioned with your old one, but also closer to the middle of what package directions might say to expect for cooking stuff.

No point going over 1100w, since then the oven is faster / more powerful than what directions say to expect. I think as ovens drop below 900w they seem more wimpy.

1000-1200 watts is the sweet spot for a residential use microwave. I love my Sharp 1200 watt MW and kept it to put in the pantry even though it was an older and not so pretty model. If I'm doing actual MW cooking that is the one that I will turn to.

I don't like super-powerful microwave ovens. 1000 watts is enough for me.

Adam's linked article has good advice regards to lower power levels. I routinely use less-than-full power for some tasks. My morning instant oatmeal is done at 30% (defrost) power for two 5 to 5-1/2 minute periods with a stir between for purpose of avoiding boilovers. Same for instant cream of wheat, although for shorter periods between stirs to avoid lumping. Heating (canned) soup or ravioli typically at 40% or 50%. Reheating leftovers also sometimes at 50%.

My current microwave has several sensor functions including reheat which operate at full power. I use the Potato, Fresh Veggies, Frozen Veggies, and Frozen Entree sensor settings. Sometimes sensor Reheat depending on what's involved. Sensor functions operate by detecting emitted moisture so covered dishes need a vent to avoid overheating or overcooking.