Alex, kudos on taking seriously the housing of these awesome animals!
As far as finishing the Pond... a good mortar mix will suffice, and you can apply it by hand...A good type S, with 12-14 heaped shovels full of sand and 3 shovels full of marble dust mixed thoroughly will give you a good result. Practice on a small area and learn a little before diving headlong into it...working time is limited..
Look into building your own pond filtration, and adding your UV sterilization inline of the outflow of the filtration, several great aquarist sites are out there. I also recomend you contact Danny Conner at
www.reptileadventures.net He you will find is an absolute wealth of experience and knowledge of crocodilian keeping.