If you look at RO/DI units they are all pretty much the same at the basic level. Spectrapure is a well know quality brand product. You mentioned 2 DI units. Depending on how much water production you are going to be doing, will decide your requirements. The reason I say that is that the DI resin breaks down and you can see it happen. The more DI resin you have will also extend the time between change outs. Its effectiveness can be measured in two ways. By measuring your TDS ( total disolved solids ) or by using the color change cartidges. I recommend measuring the TDS so that you are evaluating your whole filter. With a 90 gal system and I am going by your info on your footer. A 4 stage single DI cartridge system will be sufficent.
The 60 GPD unit SC has would be a good start at 210 bucks. You can always add on the gizmos, i.e. pressure guage and electronic TDS meter later if you don't want to spend that extra 100 now.
In reference to your question about the units on EBAY. The filter canisters are all the same design. Now, I haven't spent much time researching the filters they use. I would suppose that the filter inserts could be of a lower grade just like you can find for your home furnace. Another item to consider with the cheaper EBAY units is that the majority of them have white non see through canisters. You need to see the filters to monitor their cleanliness. The see through canisters are more expensive.
On the flush kits, I really don't know how much good they do. The reason I say that is you can't see inside your RO membrane. I have no way to gauge wether it really works or not. I have one and periodically turn it on, but there is no obvious difference present.
Just a few things to help you out.
