Is it possible to have full use and range of motion after knee replacement? Will my knee be just as good as new after knee replacement, or will there be less range of motion or other limitations? .

Should improve. The majority of the time your pain will get better. Your range of motion is at least to 90 degrees and many times past 120 there are however many complications which may occur with knee replacements.
Not 21 again. The goal of a total knee replacement is to relieve pain and improve function. It is not the knee you had when you were 21, however. The amount of use and range of motion you can expect after surgery is often determined by how aggressively you do your post-operative rehabilitation. Talk to your orthopaedic surgeon. See a physical therapist before your surgery for "prehab" therapy.