Test your current water source or download the info from your supplier. Knowing your home water pressure, temperature & incoming TDS will all be great info in helping to decide what unit to buy without going overkill with extra stages & unnecessary options. Eg, if your house pressure is at least 65 psi or more, you don't need a booster pump. Your house may have a PRV (pressure reducing valve) in the plumbing and these are adjustable within a certain range. You may be able to bump up the pressure in your house by tweaking the PRV. If you consider this, be advised that you'll need to ensure all your plumbing downstream of the PRV is in good condition. Most home plumbing is rated for 200 psi or better, so really shouldn't be an issue.
As far as changing your system, you'll find that most RODI units used in our hobby are built using standard 10" filter housings and regardless of manufacturer, you can usually add on or remove stages quite easily if your source water changes significantly as in the case of a move elsewhere. Booster pumps can also be added later no problem.
A good starting unit will have at least one pressure guage & a dual TDS meter, one sediment filter, a good quality carbon block, RO membrane & vertically mounted DI stage. You can add on & expand from there as needed.
Here's a link to FAQ at BFS. I did not buy my system from them, but I've seen that they consistently give good advice on many marine forums.
http://www.buckeyefieldsupply.com/FAQ.asp#FAQ21