First of all, you have my congratulations for your new arrival and condolences for the grief of uncertainty you're experiencing.
A lot of your questions you should be asking your doctor. This isn't easy to do when he first gives you the news because you are still processing the shock of it. Make an appointment to discuss it with him. Bring paper to take notes and your spouse or another adult you trust. They will hear things you miss and help you process it later.
I'm not a physician, but from what I've read, symmetrical IUGR can have a number of possible outcomes depending on the cause and severity. Again this is my non-expert opinion, but I take the fact it didn't seem to be present before week 34 as a positive sign.
Developmental delays are something I do know about. My daughter was born at 26 weeks. As hard as it is to hear, there is no way to predict at birth how things will turn out. In my experience with my daughter and other acquaintances, if your child is going to have developmental delays, you will usually be able to tell within around 6 months to a year. My daughter wasn't diagnosed with cerebral palsy until she was around a year old. Even after a year, the rate of advancement is very unpredictable. My daughter was often ahead of her peers in some areas and way behind in others. After 7 years, we have more of an idea of how she will progress, but she still occasionally surprises us with some sudden advancement. Many children have very difficult childhoods, but are perfectly fine by adulthood.
You have some difficulties ahead, but I'm confident you will rise to his challenges and enjoy your child's uniqueness no matter what. Best of luck to you.