First thing to consiter is what capacity of RO
filter you need. Once you determin if you want a 25/50/75 or 100 GPD unit then you can think about the number of stages. 2stage, 3stage, 4stage, etc.
units you can check out a few of our sponsors at
or you could look on ebay for some bargins.
Sure you can use a 3/4" female to 1/4" male compression fitting adaptor. This is what i use for my RO
hookup. You should make sure your fitting has a good washer with it and you should get some teflon plumbing tape to seal up the threads.