As for rod length, typically if you are fishing in tight brushy situations, a shorter rod is desirable, i.e. under 9'. For beach fishing, or larger rivers/lakes, a 9' is the norm, but you could even go to 9'6" or 10'. Probably the most common and the most versatile is a 9' rod.
As for rod weight, it is all dependent on the fish species you are chasing. It is a balance between enjoying the fight with the fish by having a light enough rod, and getting worked over by the fish by having too light of a rod. It is also dependent on the fishing conditions you will most commonly experience. Windy conditions, large flies, and long casts will necessitate a larger weight rod. For small stream trout and bluegill, I would go with a 4 or 5 wt rod, and for stripers, bass, and Alaska salmon, I would go more for a 7 or 8 wt rod. If I had to chose one rod for your target species, I would probably opt for a 6 or 7wt. The two situations are almost too far apart to settle on one rod, but it can be done. It is like playing a game of golf with only one golf club, it would be kind of strange and not very ideal, but it could be done. You won't notice much difference between a 6 and 7 wt, or a 7 and 8 wt, just a little stronger backbone with the higher rod weight, but there is a definite noticeable difference between a 6 and 8 wt rod. It has been my experience that most fly fishermen, when going to multiple setups, tend to skip numbers. For instance, I have 4, 6, and 8 wt rods, and my dad has 5, 7, and 9 wt rods. That is unless they have a ton of money to throw into their sport or they have very specialized situations.
Sage is definitely a great company, and they make a darn good fly rod. Other good companies include: Thomas and Thomas, Winston, St. Croix, Orvis, Temple Fork Outfitters, Echo, etc. Besides how the rod casts and feels to you, probably one of the most important aspects in rod selection is the company's warranty. It is good to know, after spending a couple hundred dollars on a fly rod, if something should happen to it, it can be repaired or replaced at little cost to you. Some companies (Orvis, TFO, St. Croix) have a not fault warranty, no matter how it broke (while fighting a fish, in the car door, or at the hands, or teeth rather, of Rover) they will repair or replace it for a small processing fee ($20-30). And with other companies, depending on whether the break occurred because of a material defect or user error, they will repair or replace at a small cost to the individual. For example, about a year and a half ago, I broke the tip section on my Sage rod, after sending it back to them and paying the $30 processing fee, they sent me a whole new tip section, despite the fact that the rod model was obsolete, and I had purchased it over 11 years earlier. I consider that to be a good company.
As to the different rod models that Sage, or any other rod maker, offers, the differences lie in the taper of the rod, i.e. the rod action, the materials used in its construction, and the time and energy spent in designing the rod. Where the rod bends or flexes determines its action. A fast action will bend towards the tip, while a soft (also called medium or moderate) action will bend further down towards the handle. Fly rods can range from medium action to very fast action. While the current trend is to lean towards faster and faster action rods, it is really dependent on the caster and his/her personal preference. A moderate action will allow the caster to feel the rod load during the cast, while a fast action rod will better allow the caster to punch a tight loop into a stiff wind.
As for reels, I have heard nothing but good things about Nautilus, and yes, the Featherweight 7+ reels can be used in both fresh and saltwater. Other good companies include: Ross, Galvan, Danielsson, Teton, Sage, Tibor, Abel, Bauer, Orvis, etc. Company warranty is definitely a consideration once again. Other issues when chosing a reel, include the anodizing (corrosion resistance), drag system, backing capacity, and overall weight. I have found that doing searches on this forum (and others) regarding a particular reel to be invaluable. A good reel needs to have a proven track record, and asking someone who owns one, regularly fishes with it, and puts it through its paces is typically worth more than any description the manufacturer comes up with.
I would definitely take the time to visit a fly shop or two. Find one you like and trust and spend a few hours (or a day) talking to them about where and for what you will be fishing and test casting a bunch of fly rods. You want to find someone who is knowledgeable enough about the equipment they sell, and at the same time won't try to oversell you on stuff you don't need.
All that being said, for me, it comes down to budget. I would love to have the newest Sage or T&T rods and the nicest Sage or Tibor reels, but with starting a family and trying to purchase a home, I have had to get a lot more creative about how I obtain my new fly fishing gear. I build a lot of my own rods now, and my reels are almost always second hand purchases.
I know my reply was super long winded, but I hope it helps.