I agree about purchasing Masters pre-30 (or even 40). Unless someone is just unloading one, why bother? You could make a case for an Adept III/Master for the pet for a pet class, but then you would just level faster making it obsolete more quickly.
As far as a scout? Dunno if I have seen a pre-20 one. Certainly there are Adept threes you can have made.
At any level, I look at the spell progression and see how soon it will be before an upgrade. I wouldn't buy anything that was being replaced soon. There are occaisions where a Master (or your Master II selection) would be better than then even the Adept I replacement, but its hard to know beforehand.
As far as a scout, I'll give the cop-out answer. How do you play? If you solo a predator, one of your damage CA's would be a good choice (even the stealthed flanking ones). If you group a bard, one of the group buffs. Just look at how you play and pick one thats used most of the time. Lots of examples...if you duo'ed with a summoner and had a predator, the aggro reducer might make all the difference in the world. If you are a bard with 3 or 4 other scouts...one of the dps enhancers (which lose a little value later) would also be a good choice.
Again, with a lot of folks at the 60 cap, there are a lot of twinks being raised, an option would be to cash in-on any rares you might have used for an Adept III or rare armor and use the cash to twink post 30 or 40.