Bare in mind... we both hadn't owned a dog for many, many years and in my case not since I was 9 years old. That makes a difference as we were so inexperienced.
If I had my time over again, I would go for a golden oldie. A golden oldie is usually too worldly wise and lacking in energy to challenge the way things are.
Any dog coming into your house will have to learn to live with both your boundaries AND you cat's boundaries.
Chester was 3 years old and came with so many behavioural issues - some of which we've never solved - that he really did take all over determination and focus to tough it out. I have so, so often wished we'd picked a dear old crock of a retriever or an little old Westie. Some of the folks here will testify how close to chucking in the towel I was at the time. If I hadn't of been so involved with rescue then yes, I would have sent him back.
But when I looked up his chip history we discovered we were his 7th registered keepers. No wonder he's a fruit cake! He's had so little continuity in his life.
We wouldn't give him up for the world now.... not until the next time he raids the dustbin and leaves refuse all over the garden, or tries to attack the postie, or tries to eat the cat's poo, or steals someone's lunch then we might rethink our policy... you get the picture?