You really have allot of problems with your shower now, matter of fact, your pan is most likely leaking, your grout and corners are leaking and you probably have rotting going on inside the walls and you have compromised drywall.
What I would do, if there is another shower or bath is to rip this thing apart and allow for drying out. Evualate the damage and make repairs once it is dry. Get circulation and air moving around the inner walls.
Now, you need to determine the location of the drain. It is prudent to find a enclosure that has matching drains. other wise you have allot of plumbing to do to meet the drain pipes up.
Most enclosures will not fit perfectly into the existing area, you will either need to build in, but in most cases you do not have the luxury of expanding. So, find a enclosure exactly the size or smaller and build in.
the installation from that point on is pretty straight forward.
PS I think with all your existing problems you are saving money and time by doing the enclosure. The going rate where I live just to replace the pan and do the concrete work is about $650 in labor. There is allot you have in the inside of a shower pan and the walls up from the pan.