Hi Callie,
I've had two underground fences. One was professionally installed, but the 2nd time we did it ourselves. Both were PetSafe fences.
Doing it yourself is so easy and a fraction of the price. You don't have to bury the wire. They sell these large staples that you can push into the ground to hold the wire in place. Eventually the grass grows up and covers the fence. We installed ours around two acres in one afternoon.
As far as the training, the professional installers are handy for that because they help you get started. Basically you introduce it with your dog on a leash and the collar set to noise only for a day or two. Let your dog walk into the area to hear the noise, then pull him back and praise. On the third day introduce the correction on the lowest level. Always be ready to pull your dog right back to you. On the 5th day, increase the correction. You keep doing that until your dog does not challenge the fence. Most do not need the correction level very high. They will learn quickly.
We were told minimum age is 4 months, so that's when I train mine.
Some of my dogs got upset about the correction and would not leave the porch for a day or two. This makes you feel very bad, but believe me, they will get over it. Take your dog out into the yard and play, give treats, etc. Let him know that the yard is a safe and fun place to be.
I bought mine through ebay. I've bought extra collars there and also at radiofence.com.
The fence will not keep people, other dogs, or predators out. Never leave your dog unattended inside an invisible fence.
Good luck!
Dixie