Gearing is important insofar as you'll be less fatigued if you can find a gear you're comfortable in. With more ratios it's more likely you will. Not so in the good old days of 3-speeds. Even the five-gear rear clusters on older 10-speeds often didn't let you find the right gear. They tried to do a 2:1 range (14-28 teeth) with only five gears. The result was that the gears were too widely spaced, especially on the high end. You can get away with a 15% or even 20% jump in ratios in your lower gears, but for your higher cruising gears 10% or so between gears is optimal. A ten-gear 11-25 rear cluster gives you all this, and even has a wider range than the older five-gear clusters.
Regarding weight, let's put it this way-I'm maybe 30 pounds heavier than I was in my early 30s, and yes, I do notice that much weight, but mainly on the hills. For example, a hill I used to take at 17 mph now I can barely take at 13 mph, although to be fair probably 1 or 1.5 mph of that is due to poorer conditioning from riding less than I used. So once you factor out the difference in conditioning, we're talking at most a 3 mph difference on a fairly steep (about 4.5%) hill, or put another way, 30 pounds makes me about 18% slower on this hill. 4 pounds would make me less than 3% slower, or under half a mph, and only on a few of the steeper hills. So no, 4 pounds would hardly make any difference at all. You and the bike would weigh roughly 210 pounds give or take. 4 pounds is only 2% of this. On a really steep hill where you might only be going 8 or 10 mph 2% is not something even I would notice. On the level the extra weight will make zero difference. I'd say for the price range you're looking at 20 or 21 pounds if just fine. Weight in bikes is like speed in racing. You can get very light if you want but it'll cost you big time, and past a point there really isn't much advantage. The biggest gain really is going from a 30+ pound steel framed $99 department store bike to the ~20 pound $800-$1000 bikes that you're looking at. That's a noticable gain even for the casual rider. However, paying another $1000 to save perhaps 2 or 3 more pounds really isn't worth it unless you're in competition. Bike weight is probably more important for a lighter person. If you were a 4' 9" 80 pound female rather than a 5' 8" 190 pound male I might say 4 or 5 pounds would be fairly noticeable as it would be around 4% to 5% of the total weight. In your case, it will barely be noticeable, if at all. Besides that, in your price range it appears the variation in weight among different models is probably only 1 or 2 pounds. You won't notice that at all.
As for shifters, the main difference between the high and low end stuff is probably durability, although the high-end parts might also function a little more smoothly. For your uses, or even mine, something like the Shimano 105 will do just fine. No need to go with anything better here. Note that you'll still have a way better shifting experience than you would back when I started nearly 30 years ago. Back then, all they had was friction shifters where you basically hunted for the right gear. Indexed shifters came along later. Eventually, you had ratchet-type shifters where you can just click to go up or down. The integrated brake-shifters on the bikes you linked to are the final evolution of this. I wish this stuff existed when I first started riding.
Yes, it's amazing how much is involved properly fitting a bike. That being said, I never was, yet I've managed to do OK by adapting to whatever equipment I could afford, and adapting the equipment to me via small adjustments. However, this isn't something a beginner is likely to be comfortable doing.
This is where it helps to be on a first-name basis with a trustworthy mechanic and know his favorite brand of beer :naughty: (or chocolate milk, in my case). Shops vary, but I never had a problem taking a minute or two to look over a used bike if someone asked.
Try one out, and see how well it handles an out-of-the-saddle sprint at 30mph/50kph or higher. 
