Achilles, what was the outcome? Did you go get your new baby?
A lot of good info from our comrades in the posts.
I use the same setup (18" 870) police with a fixed choke. I have had great success with 00 buck and 1 oz slugs (on paper
). However, it has been with a very specific round. I have shot all manner of buck and slugs in it, some very very hard recoiling in 2 3/4 (I don't mess with the 3 for self defense loads). The Remington reduced or managed recoil slugs and 00 buck are excellent balance between power and the ability to get an accurate second shot off. The wife can easily shoot them as well. At 100 yds (ridiculous range I know) I have hit a silhouette mid section with an aim point of the top of the head.
The only other piece of advice I would offer is the 18" for self defense has to be "aimed" not pointed as you would do when hunting/ wingshooting. So practice, train and shoot a lot. I hope you enjoy your gal.