well 4.) will become irrelevant very quickly when you proceed in the game, because if you are a merc you don't earn your income through wages my friend !
You use your merc status to do the following:
0.) (Also Before becoming a merc) u recruit all merc companions and build a nice "Gard"-unit - so anything less than 35 bandits won't harm u anymore - so that you can
1.) raid enemy caravans - easy target for even the smallest merc group (nice items they drop, which u sell for 2-3.000 denars)
2.) Plunder enemy villages (if u so desire) ! It will make you rich (allways gets you at least 3-5.000 denars) and most times there is allways at least 1 village rdy for plundering
3.) no matter who you fight - pick up every glaive, sword and take every warhorse if u got the chance - because of this mechanic (if u don't know it, it's a 1257ad feature);
You can easily pick 2,3 valuable items from the battlefield you can then keep, whilst keeping your original gear also - means the items (only when picked up - before battle-end!!!)
will then appear in your inventory - this way you can find nice swords and horses for you and your companions without spending 150.000 denars
And if you geared yourself up -then just sell that stuff and get rich !
4.) If you are powerfull enough (70-120 man) try to fight weakened enemy lords ! - you will get renown, much experience, can capture the lord, what bigger parties of your side wouldnt be able to, because to slow etc. -> weakens enemy because it freezes a lord + a bunch of thousand for you as ransom !
From there on you won't have any problem to get more and more wealthy, of course you have to win the battles you pick and so on ...
Well become a merc is somehow the easiest way in this game to get rich very fast - for example in my actual game (like, 2,3 revisions old, don't know exactly) i made 430.000denars in the first 50days of a complete new game
So my point is - wages (about 3k a week) don't count after the first 2 weeks imo. And ingame financial mechanics are fine I think. Because if you are settled and maybe have your own kingdom you will find it very much harder to finance your kingdom, because you can't just always raid villages in war, most times you have to fight !
So a mercenary maybe even richer than a king in 1257 ad ^^ ? (only got a kingdom in the old 0.97beta