Lots of questions,
You can hook it up directly to the pipes under the sink. I forget the specifics on this, but a lot of people do it that way.
You can turn it on and off easily. IIRC, The fittings that go right on the pipes have an on off valve/tap thing. You can also install simple $5 john guest ball valves in the input line to turn it on and off. RODI can also be set up with autoshut off kits, these use a float valve in the holding container and a pressure activated shut off valve on the RO unit itself (container fills to the float valve an shuts it, closed float valve creates back pressure on the output of the RODI causing the shut off valve to shut the RO unit off)
You can leave the water in a holding barrel, no problem. Keep a powerhead in there to keep the water airated and to avoid having bacteria growing on the water surface.
There is an alternative to RODI, but not very common as far as I know. You can set up a just DI system. To do this you want to use 2 seperate DI resins in seperate filters. By using the 2 part resin (instead of the usual "mixed bed" DI resin) you'll be able to recharge the resins but I'm not clear on how difficult/labor intensive this is. Otherwise using just DI mixed bed resin would be way more expensive that RODI. IIRC, the biggest advantage of the just DI system is that there is no waste water. (with RODI you will waste about 4 gallons of water for every one gallon of product water).
Generally, RODI setups are universal in that the membranes and filter cartridges are interchangable between brands. BUT - not all membranes are created equal. Filmtec membranes are top notch, others may be questionable to crappy.
Good RODI units can be had on e-bay, but there are also ones that come with junk membranes exct. Ask around. There was a thread here about one of the good ones in the last few days.
Main differences between units other than quality of the membrane;
# of stages. 4 stage systems work fine, 5 stage have an extra, more course (5 micron) first stage sediment filter which makes the fine sediment (1 mircon) and carbon filters last longer saving $.\
Membranes come in different GPD ratings. Filmtec 75GPD are very nice because they produce very high quality water (high rejection rate) and do it fairly quickly. Personally I see no benifit in getting a slower rated membrane as it saves very little $ to get the same water but much slower. If you go higher that the 75GPD the water quality goes down some (at least with the filmtec membranes).
WWW.airwaterice.com is probably the most popular RODI supplier that I know of. They have great prices, great customer service, and generally blow away most competetors. Even if you get your original RODI unit elsewhere, they are probably still the place to go for replacement filters, membranes and whatever else you will need over time.
The Typhoon III unit comes with everything you would want (5 stage, pressure gauge, bypass/flush kits installed, autoshutoff valve, and IIRC a TDS meter) for a great price.
Did I miss anything?
(but hey it was free) I was looking at the 3 pack of pre-filters that the site mentioned sells and was wondering if these fit the kent unit and if they are all I would need? I don't have a tds meter so I don't know what I'm currently putting out either. 
