I would buy new filters and a new membrane but that might equal the cost of a new RO system.

At minimum you want to sanitize the RO. That entails removing all the filters and the membrane. The process should be covered in the RO's manual.

The water in the tank is contained in a membrane and if that is not leaking then you're OK.

If the only problem with the tank is cosmetic then you can use it. It will also need to be sanitized.

The filters which is the Post and Pre Filter, and the carbon? The membrane is the hard plastic shell that contains the Carbon right? If so I can get the filters, carbon and membrane for $108 (including shipping). I looked at the online manual and Im a little confused. It says that prefilling it will sanitize it, but it also says it's important to use a sanitizer. Think you can decode this for me? lol Once again, thanks for the info. It really helped me out. This is what it says...

E. Prefill And Sanitize The Storage Tank

Prefilling the tank is always recommended so there
is pressure to check for leaks and several gallons of
water to flush carbon post filter. Tanks are furnished
with a special disinfection capsule which sanitizes
the tank when it is filled with water. An instruction
tag will accompany the tank. It is important to use a
sanitizer when prefilling the tank so the solution can
sanitize the tubing, fittings, and faucet at the time of
installation and startups.

A) Insert free end of orange feed water tubing into
the âÂÂPush Inâ fitting on the storage tank.
B) Open feed water valve and tank and allow to fill
(about 3 minutes).
C) Turn off feed water valve and tank valve and set
tank aside (15 minutes minimum).

NOTE: If an alternative storage tank is used without
a disinfection capsule, it should be sanitized with
household bleach (5.25%). Use 1 ml bleach per gallon
of tank capacity.

Here is a link that might be useful: Manual

You have a 4 stage RO...

Pre (sediment)
Carbon
Membrane
Post (carbon)

The procedure for sanitizing an installed RO is on page 14 and 15. If you're confused call them up BEFORE you mess something up.