The answer to the "buy a new membrane" or not is 'maybe'. If the unit was stored dry ... then yes ... you will need a new membrane. These 'poly-something' rolled sheets are susceptible to cracking if exposed to air for a while. If the unit was stored with water but not biocide in it ... you may be able to clean the unit of the biofilm and disinfect the membrane.
The best way to check is have them mix up a batch of saltwater (with marine aquarium salt) and operate the unit for a gallons worth. Then have them take a TDS (Total Dissolved Solids) meter and take a reading. You can then judge if the unit is still operating before you buy it.
One note though, if I was going to stake my survival on a used RO device, I would want to spend the extra $300 and be assured of a new unit and then service it myself according the manufacturer's suggestions. RO membranes can be temperamental things and its performance and ease of maintenance depends on the material used to make it.