you dont need the catfish yet, wait a few weeks, maybe even months until the algae in your tank builds up enough for them to snack on. as for the Knife-Fish it should be less than 10 dollars at Petsmart, im pretty sure they are around here.
sorry i cant help you with plants, everything in my tanks is either made of plastic or silk. plants require special care so you many wanna try plastic plants as your first starting out.
ps. i dont mind answering any questions you have haha
to my knowledge the Glass Knife-fish may grow to be over a foot long. meaning he probally wont be to confortable in a 55gallon tank. you could most likely do multiple african brown knives if you wanted to.