The type of membrane you get is of upmost importance. Most if not all of the ebay RO units use a cheaper membrane that requires 65psi(at 77 degrees is it?) to work correctly. If your not getting 65psi, your unit is not working at near the efficeincy it should. Dow filmtec r/o membranes are rated at 50PSI to achieve the same purity output. 65psi is on the high end of what most homes have for water pressure is my understanding, so unless you have actually checked your pressure, dont assume that the 65psi one will work. If your membrane is not actually removing the 98 or 99% of the TDS, then DI and filter chambers will remove the rest. If say you are only removing 90% of the tds(because your water pressure is lower then 65 and your temp is far below 77), then your di/filter chambers are going to be working 10 times harder(1% vs 10%), and will become saturated with tds and need to be replaced much sooner.
Also, if you go with a dow filmtec ro membrane, get the 75gpd one. Its rated at 98% removal, while the 100gpd one will only remove 90%(thus burning out your di/filters much quicker).
That having been said, I got a unit from airwaterice and am very happy with it. The tyhpoon 3 comes with al the bells and whistles I could want(tds meter, water pressure gauge, asov,several bypasses).