In my experience the cheaper costing ones you can get on ebay probably are just as good as a "sponsor"/"brand name" unit. They all work the same way, use more or less the same filters, same membrane, same deionizing resin. I've seen some systems costing a couple hundred bucks, which makes no sense, there aren't that many extras that make it reasonable to be charging that much. Now that being said I don't know specifically what some of our sponsors charge, but IMO there's no reason to wait for a group buy on something like this, when you can get units on ebay in the 100 range.
Ok that rant out of the way here's a review on some nice extras to have.
Unit - obvious duh, doesn't matter all use membranes from the same couple companies, filmtec and dow (I think), 75GPD is a good unit to use, it has higher rejection rates than the 100GPD (or did a couple years ago, I'm unsure if the technology has changed however). But like I said, eBay a hundred bucks or so, or go the sponsor route, but don't pay too much for junk you don't need or is a downright lie (zero-rejection unit)
Pre-filters, you'll probably be good with 5 micron size for everything, 1 sediment 2 carbon blocks, if you have a lot of gunk in your water you might want to get another chamber and throw a 1 micron sediment filter inbetween to catch stuff. Your carbon blocks you want to use for chemical absorption not particulate matter catching (although they will do that as well).
IMO don't waste your money on "chlorine guzzler" type filters, many will tell you they're really easy to get clogged then all that extra money is wasted, as to capacity the smallest ones are 6000 gallons, now before you think you need the more expensive ones with more capacity I did a little math.
If I change 30g every week, with a 4:1 reject ratio, that's 150 gallons of watter every week, which translates to 40 weeks worth of water changes, now most people say to change them out every 6 months (as a safety) so the smallest capacity (and ironically cheapest) will actually be the best price value.
Pressure gauges- Not necessary, but VERY handy, and a cheap add-on. Put one at the input to the whole system, and one on the output of the pre-filters. If both read the same, all is good, if the one after the prefilter has dropped, then it says you're getting/have a clog somewhere, it might be time to change your pre-filters. Plus if you get a clog too early, it allows you to figure out which filter has it, simply remove one at a time until the pressure goes back to being equal. If you have your own gauges, the John Guest fittings most likely will cost you more than the gauges to hook them up
Booster Pump Not necessary, but oh christ this is a lovely lovely add-on. My water is 40psi at the pump, pretty much at the minimum for it to work, plus less production, more waste. With a booster pump it cranks it up to 70psi! Now if you already have high pressure probably a waste of money, but if you have mediocre pressure, or hell want to increase it even more a nice add-on, probably one of the more expensive ones though. However the price on these has dropped considerably lately, I got mine with a solenoid + pressure valve (which I replaced with a floatswitch) for around $90 (I think), and that has been worth every penny, but that's because I had crappy water pressure to start with (live on a hill, where the water plant is down hill from me).
Deionizing Add-on, I list this separately because there are many systems with out this, and there are a couple options. Typically an all-in-one unit comes with a little DI chamber, which is fine, but for serious production you'll want at least two said chambers, or a separate canister with DI resin.
In line TDS, again not necessary, but very useful in telling when your resin is exhausted. Sure they have color changing resins, but to me it's really hard to tell, simply because two different resins always separate in the chamber from the get go, so I can't remember which is supposed to be what color. A hand held one can work too, but there's always some residual crud in my container (garbage can), which can throw off the value a bit, and that little tds isn't going to kill a tank, so the reason for the meter is to tell me when the resin is exhausted.
Hose Do you have a backyard of any sort? Any trees on your property? Run a long length of 1/4" hose from the waste water output to your yard (trees are nice because they suck up more water than you think). Hey you paid for that water, why throw it back down the drain, it's probably cleaner than the water from your faucet! And never have to water your yard again. However don't get to creative with drip lines and the like (put nothing that physically restricts the flow), leave the ends open, however teeing off to multiple parts of the yard should be fine.
WHEW, ok long winded advice there, but I'm proctoring a test now, so I have nothing else to do.