As a spy, you're best off spending your time working to assassinate the opposing medic. Since the other team has only one medic, it is a huge advantage to your team if you can kill (or badly hurt) the other medic, even at the expense of your own life.
Since this is a competition-level play, you can expect full communication from the other team. You will very rarely fool them into thinking you're on their team. They all should know where everybody is. (Whether or not they really do is another matter). The simplest way for you to be effective is to find a nice sport to Cloak and Dagger, wait for the enemy medic to run by, and revolver or ambassador them from a distance.
Mix up your spots, be unpredictable, and stay hidden. You may also want to spend some time hounding the enemy sniper, depending on his skill level. I've seen highlander teams completely fall apart from a skilled enemy sniper. On the off chance he's razorbacking, the ambassador will be the weapon you want to go with. Mix your disguises, but definitely use the sniper disguise - that class matches closest with this play-style.
I'm pretty sure cosmetic refers to Hats and Misc items.
To my knowledge items are only referred to as rare on the new crafting screen (Screenshot).
At first it looks like they describe the same as cosmetic, but I think they mean the 5 set hats which are not only cosmetic, but actually have an influence on gameplay.
Following by that, I would assume free accounts miss the blueprints for headgear.
There's also the special category in the crafting screen (Screenshot) which currently contains blueprints which require special ingredients that could only be acquired during the Halloween event.
Best Answer
The Hetman's Headpiece is a promotional item that's awarded in Genuine quality to players who pre-ordered Mount & Blade: With Fire & Sword. Later it would be made available in the Unique variety via crafting and the item drop system; however these are always craftable (aka clean).
Uncraftable (aka dirty) promotional items such as the Hetman's Headpiece are not supposed to exist, but technically can still be obtained either through glitches or some other obscure methods. Therefore very few dirty Hetman's Headpieces (and certain other promos) actually exist and as such, any of these that do exist are extremely rare and valuable, at least to collectors. According to stats.tf, as of this writing only 27 uncraftable Hetman's Headpieces exist out of a total of almost 30,000 of this item in total. That's less than 0.1%.
If an item is widely available as an uncraftable item, then it will very likely be less valuable than its craftable counterpart.