I'm a noob so I don't know if my opinion counts for much but I'll give it anyhow. I started a few weeks ago, and bought that exact same kettle (15 gal. from MoreBeer). I had the exact same thoughts as you, wanted to make sure I could move up to 10 gal. batches later if I wanted, without having to re-buy stuff. I've only done two batches so far with it, and they were both 5 gal. batches. But I'm still glad I bought the big kettle. It boils quickly, and its really not that hard to handle. I'm using a CampChef propane cooker (also from MoreBeer), and I think that makes it a lot easier than trying to wrestle it up on to the stove top. One thing though it does boil off more water than I expected, so plan on figuring out your own boil-off rate and plugging that in to iBrewmaster or whatever program you log your brews with.
The position of the valve is just fine. The kettle comes with a length of hose to attach to it, and I can drain it right into a carboy that's sitting on the ground--the kettle is the perfect height for this sitting on the CampChef. Yes it leaves a lot of liquid in the bottom (I don't have a pickup tube), but I found I liked it that way. As you get to the bottom and below the level of the spigot, you can easily tilt the kettle to get the last of it, and easily control it to ensure leaving as much of the hops and crud in the kettle as possible.
I do have one piece of advice if you go with this kettle. Choose MoreBeer's 5/10 Split Wort Chiller. I didn't specify that one, and they sold me the regular one that normally goes with that kettle for 10 gallon batches. As some have said in this thread, it holds most of the coils up too high out of the wort on a 5 gallon batch. The 5/10 one is designed to keep the lower half of the coils fully submerged in a 5 gallon batch. But even with the not-optimal chiller, I can still chill a 5 gallon batch to pitching temp pretty fast. I gently pushed the coils a little lower on the cooler to try and get more efficiency out of it.
I think you're on the right track!