Go with your instincts
If you feel that this puppy is right for you, then you should go for it. As long as the puppy as healthy and playful and happy
Just keep in mind (and it sounds like you are) that it's not only that these smaller dogs are sick more often than bigger dogs, it's that you also have to be more careful with Everything. If a tiny dog eats something it's not supposed to, it will take a bigger toll on his body than a bigger dog. If you step on this dog accidentally, it could hurt him easier than a bigger dog. A dog that size should not be jumping on and off the couch or bed. He/she is likely to break a leg easier by doing that. You will need to get doggie stairs for the couches and bed...
I have a 3.6 lb maltese who is 11 months old and she is very healthy, but I'm very careful with her. We shuffle our feet when we walk and we have to look behind us before we take a step back because she's always at our feet. We have stairs for her to get up/down from the couch and bed.
And we also have an 11lb yorkie who is 3yrs old
He weighed 1lb 2 oz when we got him 12-13 weeks old and he was very sick, but we didn't know it at first... He was diagnosed with liver shunt/mvd when he was around 9 months old.
Make sure and take the puppy to the vet the day that you get him or the very next day. Have the vet check for luxating patellas, open fontanel, fecal test, etc.
Good luck and let us know if you do get the puppy
We would love to see pictures