Help me understand RO filters, membranes, resin...

I've had a WaterGeneral RD-106 RO unit for years. I can't remember the last time I changed the resin or filters or anything but I know it's been several years. I'm getting a new tank and I want to make sure I start off on the right foot with everything. I also have been suspecting that things get much worse in my nano after a water change. Is it possible my RO water has gone bad and is actually hurting the tank by not replacing stuff? I know the membranes "should" be replaced every couple of years but does that hold true if I only use less than 5 gallons a week? What about the resin? What's it's purpose? Just for taste? If so, does it need to be replaced? Lastly, can I buy any filter and any gpd membrane or should I stick with what came with the WaterGeneral unit? I definitely don't need 100GPD which is what comes stock with the RD-106 but I don't know anything about the flow restrictor. 

Thanks Bryce. I've been researching all day it seems and I still can't decide if it's better to go with BulkReefSupply for about $30 more overall or go with a different brand I found on another site. How much does brand and quality play into this?