Yep... It won't actually take too much to change the upkeep to 0 for all units, there ain't much in the vanilla EDU somewhere around 200 I think... all thou need is patience, I recently managed to rebalance all stats and unit prices for all units in 10 hours

. I'd rather suggest a better gameplay with modding the recruitment time to 0 instead of upkeep. Removing upkeep removes the difficulty, it's like cheating (Pressing "~" and typing add_money 40000 as many times as you want to add money). Well... that's for upkeep.
To add assassins , spyes , diplomats , characters here's what you have to do:
1)Go in the game, choose a position for your diplomat, move the cursor over it, press "~" (to activate console) and write show_cursorstat. Take a pen. Write as many coordinates as you want depending on how many agents you wish.
2)go in bi\data\world\maps\barbarian_invasion\descr_strat.txt or data\world\maps\imperial_campaign\descr_strat.txt (if for vanilla RTW).
3)search for (pressing CTRL+F) "romano_british" and go at the end of the faction descriptions but right before "character_record" is seen.
4)there you add the info as it is for other factions
"character Decius Curtius, diplomat/spy/assassin, age 29, , x 103, y 73 (coordinates noted from campaign in step 1)
traits GoodSpy 4"
You will have to open bi/data/descr_names too and choose names from there depending on the faction you wish to add an agent too (You can't add Decius curtius as a romano-british agent since the name doesn't exist in the database of romano brits, to go straight to romano british use CTRL+F and search for "romano_british")
Farewell