To get seeds the plants must have reached the seed setting stage and they should come from disease free stock
This is no guarantee that the dreaded downy mildew will not infect them during the growing season
I notice that all the big growers-like Jersey Plants- were selling plants last year and are again this year
So it is just a case of you taking a chance-unless they say on the packet "downy mildew resistant"
What about sunpatiens? Are they resistant to downy mildew?
New guinea are.