Doggie Dilemma!

[TsPoet]

Shelter dogs have baggage. All dogs will need trained. Especially for a shelter dog, I'd suggest CGC training.
In general, dogs have a 'fear period' between 6 and 10 months of age, so be aware that at that age, his/her personality may change and you will have to deal with a fear period.
I have 2 shelter dogs, both with issues. I became a dog trainer (volunteer with a club, not professional) due to having to deal with them. i also hate to admit it, but I now have a well-bred pure bred dog from a very good breeder. I was too tired to get another shelter dog.
If you go with a breeder, avoid a back yard breeder and a puppy mill with all your might. PM me or post more questions and I can help send you down the road to a good breeder - generally, though, you need to pick the breed or a few breeds first.
Shelters are pretty good with honest evaluations of their dogs, an older dog, say 1-2 years will have been evaluated by a rescue and you may have a better idea of what it will be like. Speaking of that - shelters are good places and you can find a good shelter - there are other types of rescue orgs as well, many work through fosters, so will definitely have suggestions for you about dog personality.
I know this has been scattered, but here is another scattered thought - while shelters guess at breeds, breed characteristics are very real things, so thinking of a breed, breed characteristics, might help pick a personality type.
What type of dog did you foster? what personality traits did you like? hyper? calm? snuggly, aloof...? Many (most, or all) people pick a dog by looks - sounds like you are smart enough not to necessarily do that - so instead go in with a list of what you like/want and ask them to help you pick.
I have an aloof, and incredibly smart, shiba inu- mix
A tough little reactive tank of a doxie-mix
and a scatter-brained, hyper, extremely fast and not very trainable, purebred Italian Greyhound
- all of their personalities are surprisingly close to "breed" characteristics, good and bad.
The last thing is gender - many many people want females because they aren't aware that dogs are opposite of humans - males are easier and nicer and generally the calmer and quieter gender (often with more potty issues), females are more apt to be dominant and domineering and more pushy. (or maybe that is more human like, LOL)