Not bad. Entertaining for a while. I bought everything and I have to agree with the "on level up" upgrades. There are two of these, one for the main character and one for the lance knight behind him. Get these two upgrades to max before you level up any of your characters. This does mean you'll be grinding out the first few caravans for a while.
To purchase everything in the game you will need to collect 21,208,350 gold. The archer's third skill helps with money making, the wizards second, and critical hits(it seems) help earn money.
Personally I don't like the skills being hidden, I would have preferred grayed out. That way I could see what I can get and immediately get excited about wanting to upgrade it.
Here is a list of all their skills in order of unlock. I suggest unlocking new skills till they get a bit too expensive for you. Skill upgrades drop off in power dramatically after the first unlock of it and progressively get more expensive and give less and less bang for you buck.
Main (Nando): damage resistance, increased damage, all robbers HP up *ON LEVEL*, All robbers +Crit chance, +stun chance, All robbers increased damage, All robbers +crit chance(yes there are two of these skills), Increased damage, Everyone but Main/Nando gets attack speed increase.
Lancer (Sir Spearlot): Increase damage, Attract drops (Useless), All robber increased damage *ON LEVEL*, Activate skill last longer, Shield block amount increase, +stun chance, +block chance, +Crit chance, Massive damage increase.
Archer (Nibor Hood): Increase damage, +attack speed, Increased money from coins only, +Stun Chance, Increase damage (yes again two of the same thing), +Crit chance to all robbers when using active power, Active skill lasts longer, Increase damage (non%, +55 dam at max), All robbers do increased crit damage.
Mage (Mr. Nile): All active powers last longer, Increased money from gems and gem coins, Make trader attack slower (damage mitigation),+Crit chance, Increased damage, Active skills recharge faster, +attack speed, All robbers increased damage, All robbers +stun chance.
Level 16 is the final boss and he hurts a bit and has 70,000 HP. Make sure to level everyone up a little at least.
I was able to get all the medals in a single play and they are active for me. Using Mozilla Firefox.