Yeah, I've already dealt with them. I'm playing on Monarch and Marathon speed. I expanded extremely quickly in all directions.
I would be surprised if the amount my mercs were costing was over 50. Really, that's only enough for one extra bump up the slider.
China is becoming frightening though. They're huge, teching way ahead of everybody, and are Solid/Very Solid. I don't think the Mongols will give them anything to worry about.
You're not on the latest version if there are still mercs.
Since you're still playing the old release version, and on a slower speed, it's like shooting ducks in a barrel.
1) Don't invade Persia from the get-go, it's not a good idea and it just makes your defense against the Seljuks harder.
It's also a bad idea to start with infrastructure immediately, as you need troops immediately to prepare for the Seljuks.
2) Expand west instead and make sure you have Greece and Rome with your starting troops + mercs + a few siege.
3) While you're doing this, just build Barracks and start to pump out Camel Archers;
if you're utilizing Vassalage or Theocracy, then there is no need for Stables; they're a waste of Hammers considering your deadline
(which is again, very generous on a slower speed). You need at least 15, all double promoted to Combat II and at least 5 Barrage II Cats.
4) When the Seljuks spawn, let the first two stacks have it. You should be able to eliminate one immediately and clean up the other one with your veteran stack from Europe.
5) Heal up and wait for the second wave. You're done.
6) Ottomans are a joke after this. Crush them, then delete all troops with no moves left from the flip zone and evacuate all cities within the flip zone.
Also, the latter (advanced China) is expected on both:
1) Non-SVN version, which you are playing; the Mongols are currently very overpowered (as AIs) on the current version, I'm positive most can agree.
2) Non-standard slower game speed, due to civs with cumulative effect UPs gaining more total benefits over a longer set of turns.
Not saying you MUST play on Normal, but if you're not playing what the game is balanced around, don't expect more, especially on an easier setting.
And certainly don't expect it to be tailored to your pacing.