First of all, welcome!
Smaller than a Grey - That's a lot of species!
Affordable overall - Depends on what your opinion of affordable is. You'll have to consider the cage, the actual bird, toys, food, perches, etc. Depending on the bird, you'll probably have to replace toys frequently as well as rotate them so your bird doesn't get bored. There's also the vet bills to think of - avian vets aren't very cheap!
Not a testy species - Cockatiels or Budgies, probably.
Not bothered by busy households - This really depends. Some species are more tolerant than others, but for the most part it depends on the individual bird, I think. Cockatiels would be good, and probably Conures since they are so lively anyway.
Not extremely loud - Cockatiels, Budgies, or Pyhurra Conures. Also depends on your opinion of loud. To some, a Cockatiel is loud. To others, a Macaw is quiet!
Is tolerant of constant attention - This is a tough one. On one hand, it is good to give your bird attention and out of cage time, but it's also good for the bird to be able to entertain itself. Too much attention CAN be a bad thing. For the most part, though, all birds like attention. They are flock animals, after all. Cockatiels love to be on you, from what I have heard. Ringnecks just kind of like to hang out, they aren't much for cuddling or anything.
Easy to potty train - Also a tough one. All birds can be potty trained. But, depending on the species... Well, a Macaw can hold it for about 20-30 minutes from what I have read. But a Budgie will probably go every 2 minutes. It will be much harder to train a Budgie than a Macaw. You also have to consider the fact that some birds get infections when potty trained because they hold it for way too long. You have to be careful. And diapers are alright some of the time, like if you're going to a friends house... But you don't want to diaper a bird all the time. It will irritate the bird's skin/feathers and it could affect the vent area as well. Birds poop. A lot. It's just something people live with.
Lovebirds are very feisty and can be nippy. Ringnecks are great talkers, but they are more hands off birds, they don't like too much attention. Parrotlets are like a big bird in a little package. They are also very high energy and feisty little guys.
Have you looked into Cockatiels? A Cockatiel honestly sounds right up your alley!
Good luck in your search!