I have this Watts Premier Pure Plus RO system and can recommend it. The big advantage of this system is that all of the filters can be replaced very easily with the press of a button. Yes, you are probably paying slightly more for the replacement filters, but I think the price is well worth the ease and convenience.
Watts Premier 531417 RO Pure Plus (VOC) Reverse Osmosis w/Brushed Nickel, Top-Mount Twist Faucet
I used to have the old-style RO where the filters are inside a big housing. Those systems have cheaper filters, but are a huge pain to replace because when you unscrew the housing, a lot of water spills out and it's hard to do the replacement under the sink. It became such a pain that I dreaded replacing the filters and stopped doing it on schedule. But at that point, there's no point in having the RO since I'm not properly maintaining it. That's when I realized I should pay to replace it with the new style above.
Here is an example of the old style RO that I would not recommend.
The plumbing part of the installation is not that hard if you're slightly handy. The biggest challenge is drilling a hole on your counter top for the faucet. If you're not comfortable, then hiring someone makes sense.
Edit: I would also highly recommend adding a permeate pump, which significantly increases the efficiency of the RO system. This pump requires no energy source.