1. There is a fairly big difference. The chance of a pop (increase in skill) is significantly higher on an 18 year old and a 19 year old, but 19 year olds are still good to train. 18's are just the best.
2. It does get fairly harder to increase as the levels go up. If his potential is good and he is young, you shouldn't have a problem increasing already solid skills, however.
3. Outside defense is definitly a good start. As you probaly know jumpshot is a very important skill for a shooting guard, and you want that to be the highest. Depending on what kind of offense you use, affects if you should train jump range. For example, if you use an offense where you take lots of threes, you want to increase his range and jump shot. If you use an offense where you look inside, you want to increase his passing and driving. The three skills you need to upgrade no matter what offense you use are handling (for less turnovers), outside defense (extremely important against other shooting guards), and jump shot (every shooting guards need to have a good jump shot).
4. It doesn't matter what his skills are if he is an 18 year old. If he's 18 just make sure his JS is at least average and his JR is at least mediocre. For a 19 year old, his skills need to be a little bit better. If he is an 18 year old or a 19 year old with an MVP, Hall of Fame, or All Time Great potential, then it doesn't matter what his starting skills are.
4.2 The price for a trainee with good potential can be high. MVP's go for around 500-850k. Hall of Famers can go for 750k-1.5m. All-time greats go for 1 or 2 million. I suggest either getting a superstar or perennial-allstar for a price you can afford.
5. When you train you aren't always going to get a pop. Just make sure your JS is always higher than JR (and maybe higher than all other stats). Also, remember what I said about what kind of offense you run.
You should be fine, though, good luck!