Unfortunately, with a RO system, you get what you pay for. The biggest problem that you will find with the low end setups is an improper filter selection. Let's start from the beginning.
Particulate filters: These remove sediments down to whatever they are rated (either nominal or absolute.) I use a 1 micron nominal and a 0.2 micron absolute. The 1 micron filter is inexpensive ($4 each), the 0.2 micron is about $27. I filter it down so low because I want to protect my carbon block.
Carbon block: This will remove the chlorine from tap water. Chlorine will kill an RO filter in short order, so a quality carbon filter (sometimes callled Chlorine Guzzler) is recommended. These run about $12 each.
RO Filter: This is the workhorse of your unit. Sediment and carbon filters do absolutely nothing to remove dissolved salts. Dow Filmtec is the typical name brand filter used for RO filtration. There are two ratings for RO filters; 90% rejection and 98% rejection. The 98% is a superior filter and usually runs about $40 (which is why you want it protected at all times by an unclogged carbon block.) I think the 75gpd membranes are 98% and the 100gpd are 90%.
I use my filter to make drinking water. Every six months, I disinfect the entire setup and change all filters (except the RO). My current RO filter is 3.5 years old and still rejecting over 99% (booster pump overdriven) because I take care of it.
You can save some money by purchasing a unit from a reputable vendor; there are three or four out there.
A pressure gauge is a must (in my opinion) as is a TDS meter.