Unless used RO Systems are dirt cheap and you know what you are doing, I wouldn't recommend getting one over a new one to avoid headaches. A 3 or 4 stage under the sink RO system costs ~$120 brand new or cheaper when on sale and it comes with everything you need to get started. New RO systems may come with the new safety improvements like double o-ring housings, tubing locking clips, check valve, leak stop valve, etc. The vast majority of the components in under the sink RO systems are generic and simply rebranded; most of the parts are universal and can be used across multiple brands as long as the sizes are compatible.
The three essential stages for a RO system IMO are the pre-filter stage, carbon stage, and the RO membrane stage. The other stages are pretty much optional depending on your needs. There are granulated activated carbon(GAC) and activated carbon blocks(CTO) for the carbon stage. CTO is better at removing contaminants than GAC in general(depends on the micron) due to the larger surface area but GAC does not restrict the flow as much as CTO. GAC is probably better suited for purposes like a shower head than CTO. Alkaline, UV, and post carbon stages are optional as well. UV is redundant if your water is from a municipal water supply as the water is already treated for bacteria. It is targeted for people getting their water from a source that has not been already treated for bacteria like a well or those that prefer not boiling the water prior to drinking. The alkaline stage is silly to me and I am not going to delve further into it. A lot of RO systems opt not to add a DI stage because the RO membrane should already remove 96%+ of the impurities and DI resin are more expensive to replace than the others. If you have the money or really adamant on getting the TDS even lower, go with a DI stage.
The main difference between lower rated and higher rated GPD membranes is higher rated GPD membranes often have lower rejection rates. For example, a 50 GPD membrane may have a 97% rejection rate(e.g. 100 TDS tap water should yield ~3 TDS filtered water) while a 150 GPD membrane may have a 95% rejection rate. However, this does not hold true for all membranes. Depending on the quality and materials, a 150 GPD membrane can also achieve 97% rejection rate. There are also low energy membranes that are able to operate at lower water pressure but their rejection rates are often much lower as well.
A flow restrictor is vital because the majority of home RO systems require a minimum water pressure of 40 PSI to operate normally; that is why you see some RO systems sold with booster pumps. When the capacity of the flow restrictor is too high compared to the membrane GPD, the pressure in the membrane housing drops, leading to greater waste water and less filtered water coming out. If the capacity of the flow restrictor is too low, the water in the membrane housing may not pass through the membrane properly, causing higher TDS in the filtered water.
Looking at that closely I see that the wastewater does not hit the post carbon? Is that correct for my system?
The waste water should not be connected to any stages of the system. It should go directly into the drain pipe.
As for the leaks, they are obvious signs of improper connections. You can try removing the connections and adding Teflon tape to the threading of the fittings before reconnecting, double-checking the compression fittings are not pinching the RO tubing too tightly, or try a RO quick-connect faucet connector instead if your faucet is compatible.