Hi catie,
don't worry too much, I think telling your boys about bubby will be better accepted than you think. Besides they are 15 and 17, I bet they already have busy lives themselves. In fact they may even be really excited about a new addition to the family. I have a 14 year old daughter from a previous relationship, and I am now remarried and 36 weeks pregnant with our baby. At first my daughter was very unsure of how she felt about having a brother or sister, but now she is really excited. I think you just need to give them time to get used to the idea. If you are as close to your sons as it sounds, then I am sure they will be very happy to see that you have found happiness in your life.
As for the dental thing, maybe you need to speak to your dentist and just let them know that you are actually pregnant before you have any dental work done. I need fillings in my teeth, but they told me they won't touch my teeth until I have had bubby. You may be ok as you are in early stages of pregnancy, but it is really important to speak to your dentist, as they know what effects the medication they are giving you will have on the unborn baby.
The only tests I had done for downs and other abnormalities were ultrasounds and blood tests. Definately nothing invasive.
Good luck