whatever system brand you choose, make sure it uses a standard dow style membrane and standard 2x10 prefilter housings. DO NOT purchase a system with proprietary cartridges.
membrane style for reference - yes i really like the DOW ones as replacements. affordable and good quality.
the big part of the RO unit thats important is the membrane. thats where the real work happens.
the prefilters and post filters are just minor cleanup stages.
generally you're gonna be either a 3 or 4 stage system, depending if you want post RO final polishing. IMHO anything more than 4 stage is just more filters you need to service. once you get thru the membrane - theres basically nothing of significance left to come out of the purified water it produces.
some systems will add multiple carbon prefilters or multiple sediment filters on and call them extra stages, just to feature creap over the next guy (pick me! i've got 17 stages and the other guys product only has 5!). however i fail to see how multiple activated carbon filters in series can be considered multiple stages of the filteration process when they're performing the exact same process in several successive chambers pre-membrane. the only true practical 5th stage is Deionization resin, which for a drinking water system is probably quite excessive - and costly to maintain (cartridges can be $30-$60 and need to be replaced several times a year depending on usage amounts)
a standard unit the stages are going to be (in order of flow) a basic sediment (probably 5 micron), a carbon block for heavy metal removal, the RO membrane for purification, and then a post membrane carbon for taste (after the bladder tank).
drinking water system should not need a ion exchange resin (deionizer filter) as part of their filtration. your water doesnt need that level of polishing/purification. its an added expense.
defiantly make sure you invest in a system with permeate pump so that your bladder comes up to full pressure and it doesnt take forever just to get a glass of water from the drinking water tap / fridge door. without one most of the auto shut off valves will kick off well before the tank comes up to full capacity and therefore highly dimish your supply and delivery capability. they're kinda pricey, but worth every penny if you ask me.
i'm also a fan of remote mounting the RO and prefilter assembly (like in a basement, etc) and only keeping the bladder tank and post tank carbon filter under the sink to save cabinet space. this will help with delivery - minimal head pressure to overcome vs it sitting on the basement floor and having to deliver water 8+ feet from its position. RO bladder tanks dont come up to >40 psi like our wells do.