Before investing any money in what you have, make sure it works correctly and will do what you need or you could be throwing good money away.
You say 2 stages? Does it just have a carbon block which serves as both a sediment and carbon/chlrorine removal filter and a membrane? If so this is not very efficient since onse the carbon gets dirty from all the particulates, sediment and colloidal materials it loses its ability to adsorb chlorine which should be its sole purpose in life.
Beg, borrow or steal a handheld TDS meter (they are only $20-$25 new and totally necessary to troubleshoot a RO or RO/DI system and it cannot be done without it) and test your tap water TDS and the treated RO TDS. If its not removing at least 95-96% of the incoming TDS then don't spend any money on it as it is not efficient and would quickly eat DI resin if you sere to add DI.
DO NOT drain the filters, a RO membrane MUST stay wetted after it has been used as does DI resin or they go bad quickly.
The reason I say it may be more cost effective to buy a new system is a reef quality RO/DI system including a full size 10" replacable 0.5 micron absolute rated sediment filter in a 10" vertical clear housing, a full size 10" 0.5 micron 20,000 gallon carbon block in a 10" clear housing, a 90 GPD specially treated and batch tested high rejection rate RO membrane (usually better than 98% efficient so longer DI life), a full sized 10" refillable 20 oz SilicaBuster reef specific DI canister and cartridge, an inline pressure gauge and a capillary tube flow restrictor is only $128 on sale. By the time you get a replacement sediment/carbon or whatever it is yours has, possibly replace the membrane and flow restrictor at $40-$75, add an add on DI for $40-$60, and add a pressure gauge so you can monitor the filter condition and head loss due to dirty filters, your total will be close to the same money or more and you will not have nearly the same system, the same GPD or the same water quality.
Buying, bartering or inheriting a used system is often not what its cracked up to be. I have helped many many people upgrade systems and rebuild them so they perform well and it gets pretty expensive compared to buying new. I went down that road 20 years ago with my first RO system which I later converted to a 75 GPD RO/DI. I had a ton invested and should have started over.
