If you have a 150+ tank, you do go through a lot of water, and depending how it's set up, going with a higher gpd membrane may be a help. You also need to recognize that the reported flow rates are under optimal temperature and pressure, and your result will vary, always lower. However, if you have truly poor water pressure, you'll need a pump, in which case you'll get closer to the "maximum" production rate of your membrane. With a high quality membrane (Dow Filmtecs are the most popular) a 50 gpd rated membrane should be all you need. It also has a higher rejection rate than 100gpd membranes (98% vs. 96%) which is helpful in making your DI resin last a bit longer, although that's not terribly expensive either.
Plenty of reasonable choices out there, and $180 may be an overly generous budget (unless you need a pump to compensate for your water pressure). All you need is a 4 stage unit, 1 prefilter, 1 carbon block, 1 RO membrane, and 1 DI cartridge. Get only standard size filters and cartridges so you're not hunting for some weird sized filter, although that's pretty rare. Get clear canisters for all 3 cartridges, except for the RO membrane which is always opaque. Make sure you understand what plumbing comes with it, and whether it will fit the water source you plan to use.
You must have a TDS meter. They're inexpensive, and the only way to determine that your water is good for use, as well as determining when filter(s) need changing. The goal is always 0 TDS for your final make up water. It is also useful to know the TDS of your water source, which will help give you some very rough idea of how long you can expect your filters to last. I prefer changing all filters (except RO) every 6 months or so, even if my output is still 0. It may be a waste of money, but when I see the deep discoloration of my prefilter, I'd rather trade it out. RO membrane should last a few years if cared for, and that is the major replacement expense.
Folks here have found units they like from eBay, Bucky Supply which is a sponsor here, airwaterice.com, and a whole slew of other assemblers. I personally picked up a flo something 4 stage 50 gpd filmtec from F&S for $120 with shipping during a sale. Plenty of people also sell equivalent units for that price without hunting for a sale.
Others will pipe in with experiences with vendors and units, but I think these features are what to look for.
Henry
...
