I go to LA Fitness. Frankly their trainers ....most anyway have little to offer. Finding a regimen to train to as well as being mindful of techniques is all right here on the net.
You are correct in that the lower body will need attention. The upper right thigh will be the lynchpin in the equation I'd think and keeping it in harmonious balance with the whole leg.
I'll start slow with light weights and many repetitions to build strength slowly. I am in no hurry. Physical Therapy and learning to walk will be first and foremost
When I and the prosthetic leg get accustomed to one another I'll go back to some circuit training.
I have not been to the gym since the morning of my accident two years ago.
I dont miss my leg a bit. I think there is a large difference in losing a limb traumatically after an accident or for instance losing it in combat as soldiers often do. I had two years to deal with a bum leg and when the battle so save it was deemed over It was almost a blessing.
I suffer from none of the emotional grief that they told me I might experience.

And...so far only a couple of minor episodes of "phantom" pain.
Arstaren....having not been around here in recent memory ...I have no idea how many right wing extremists might be hiding in the shadows around here. I expect there are a few.

And of course there are a few out and out left wing extremists as well.

Please note tongue planted firmly in cheek.