I found out the sex at 18 weeks and they are pretty firm on the sex ( we are doing a reveal so only my mother in law knows for now) I also didn't have my first ultrasound till 18 weeks because I didn't know I was that far along. I also just had blood work done at 18 weeks and 3 days to determine if there are any birth defects so I think depending on your doctor you should be able to find out all of those things! They didn't do all of this when I had my first son 6 years ago. GOOD LUCK! and I hope you get your answers and congrats!
Now is a good time, any time going forward. I had my scan at 20 weeks. I chose to not find out the sex, but I could have. And at that time, they looked for signs of birth defects. So, any time is good!