I did Revs in 2010 and I can honestly say it's an amazing course. It does require quite a bit of work, but it's got a fascinating narrative and complex political changes so it should be worth it!

I only ever did History Unit 1, and Revs is slightly different to that. How hard the SACs are depends entirely on how hard you teacher makes them. Some teachers allow cheat sheets, others don't: that can make quite a massive difference in the difficulty of a SAC (mainly because there just simply are so. many. dates.)
In terms of relative difficulty of your specific revolutions, it's the general consensus that the French and Chinese revolutions are the "tough" ones while the Russian and American are comparatively simple. So you've got one of each, which is always a good mix. I did French and Russian myself, so I can't comment on the Chinese revolution, but the Russian Revolution is pretty straightforward. You should be able to divide your study into:
(i) Pre 1905 - October Manifesto
(ii) Post October Manifesto - WWI
(iii) WWI - February Revolution 1917
(iv) The Provisional Government/Petrograd Soviet - October Revolution 1917
and finally, in Area of Study 2 (which is a tiny Area of Study, really)
(v) October Revolution - Death of Lenin 1924.
I found for myself that fragmenting the chronology into these chunks made it much easier to handle. I did the same thing for the French Revolution, you'll find you can do that yourself for the Chinese (though both the Chinese and French ones are far more complicated and will require more than 5 dot points).
Things you need to know about doing Revs:
1. It's structured around a narrative and a chronology.
(This is as opposed to a more general, social look at history which you'll find in the Renaissance Italy subject) Thus:
2. Dates are important, and there are lots of them.
3. Following on from knowing the actual events, you will need a comprehensive knowledge on the wider significance of those events.
4. As part of getting to know the "story", so to speak, you'll become familiar with the leaders of both the old and new regimes (pre and post revolution) really well.
(e.g.) In terms of the Russian Revolution, you'll be familiar with names like Tsar Nicholas II and Stolypin for the old regime, and Kerensky, Lenin and Trotsky for the new one.
5. As an extension of that, you will need to understand the ideas behind each leader.
(e.g.) Tsar Nicholas II firmly believed in a monarch's divine right to rule, while Lenin was obviously inspired by his own version of Marxism.
The phrase, if I recall correctly, is "Ideas, Leaders, Movements and Events" which will be important in Area of Study 1 in whichever Revolution you study.
Doing well in SACs and the exam basically requires you to understand and know (memorise) content for those 5 points above, and be able to articulate your interpretation of them.
