It all actually depends on what you plan on doing w/ the sump? Do you plan on building a fuge w/ it? Are you storing a skimmer in there or does it sit outside the tank? In regards to removing the Bio Balls, how long have they been in your tank?
If you want to remove the BB its a long process... I would take out maybe 5 a week untill there all out then you can go w/ the LR or what ever else your planning on adding!
How much do you want to spend.... some poeple will use old acrylic tanks which will cost more IMO others will buy big rubbermaid containers and connect them through bulkheads! It all depends on what you want to do...
Broomer actually has a great setup w/ rubbermaid containers.. I took some of his ideas and put them to mine... for a 300 i would think two 40 gallon rubbermaids would be fine.. one being a fuge and the other being the sump...
HTH