Just from looking at the image, the DI resin (far left canister) looks spent. Typically almost all color-change DI resin I've seen, starts a darker green color, and turns that brown-gold color when exhausted. And the sediment prefilter (far right) looks oddly colored. They start white, and hence their name sediment filter, blocks dirt/clay/etc, so they almost all turn an orange/tan muddy color. Not sure why that one's green...
At this point in time I would order all three pre-filters (1 micron sediment, and 2 carbon), and new DI, they sell either a whole canister or loose resin you can pack yourself. And test the TDS reading on the RO membrane. Measure incoming TDS to the unit, and then TDS coming out of the RO membrane (before the DI stage). Look for a rejection rate of at least 90%. Meaning if you have an incoming TDS of 300, your RO out should not be higher then 30.
As for costs, yea typically the price of a new unit vs the costs of a used unit + new filters is negligible. You paid $75, new DI is $15-20, and all three prefilters will be about $30-$45. If you have to get a new RO membrane, it's about $45. These are online prices, LFS's will be more expensive. So I guess in the end you'll save some money.