Imo, in terms of health, you should be going for a ‘natural’ looking breed - no squashed faces, no domed skulls, no shortened noses, no overly long backs, no stupidly small legs.
Ideally, the breeder will have health tested the parents eg Patella screening for toy breeds who are prone to patella luxation, DNA tests for PRA (progressive retinal atrophy) for poodles etc.
Each breed will have a list of recommended health tests.
In practice, unfortunately, I find the vast majority of breeders, including those Kennel Club registered, either do not health test or do the bare minimum.
If the litter is Kennel Club registered, check the inbreeding coefficient of the litter, you want a low level of inbreeding.
Toy breeds typically don’t leave until 10 - 12 weeks, 8 weeks for bigger breeds.
By law the puppy should be microchipped, they may have had their first vaccine at 8 weeks or if at 12 weeks they may have had both.
Check the parents temperament is nice!
Cannot emphasise this enough.
Temperament is largely inherited.