Browse some of the RO/DI units on these websites before you buy one from anyplace. You'll get an idea of costs and features.
http://airwaterice.com/
http://www.thefilterguys.biz/
http://www.purelyh2o.com/
DOW Filmtec 75 gpd membranes are not hard at all to come by. You'll be better off with a 75 gpd RO membrane as it has a higher rejection rate (removes a greater percentage of stuff) than a 100 gpd membrane. In turn, the DI resin will last longer, you'll buy resin less often, and save a little bit of money in the long run.
It is better to pick a unit with a verticle DI chamber, as there have been reports of issues with horizontal DI chambers. Regardless, the vertical DI chamber allows for easier replacement of the resin (my opinion), as it's available in pre-filled cartridges or as loose resin that you can use to refill a refillable cartridge. The DI resin can be bought in bulk, which is another opportunity to save some money over time.
Having your own TDS (Total Dissolved Solids) meter is very handy, but not entirely necessary from the start if the budget is tight. It will help you keep track of how well your RO/DI unit is performing and is a more reliable way to know the DI resin is exhausted than waiting for it to change colors.
I would, however, recommend having a pressure gauge. I've found it very useful to have one installed just prior to the RO membrane (ie after the prefilter cartridges) since it allows me to monitor the water pressure getting to the RO membrane itself. This means I can tell by the pressure readings if my prefilter cartridges are clogging.
Good luck!