Not recommend you buy from US.
First, US iPhone is locked set. Which means can only use with local network (AT&T).
To use in Malaysia, you'll need to jailbreak and unlock, which will cause you some trouble.
And, you cannot update the firmware as you wish, as updating will cause the device being LOCKED again (and you need to unlock again).
No international warranty. If you buy at US, you'll have to claim warranty at the country you bought the iPhone, which is US.
Save your trouble, get local set (DiGi or Maxis).

And please, read before post. The answer is already given in this very same topic, first page, section
2) The Difference Between Unlocked Vs Locked Units

Sim free means you can use the iPhone with any telco. Like the one sold by our local telco (Maxis & DiGi) is Sim free. You may use any sim card you want. They are official unlocked version.

Yes.
Sim Free = Unlocked Unit
Sim Locked = Locked Unit

I don't know about the iPhone sold in US nowadays, but still recommend you get one from our local telco. Warranty matters.

16GB or 32GB is very personal questions. That would depend on how you gonna do with your phone.
For me, I used it for listen music, online, install mini/puzzle games, taking photo (not much).
16GB is far enough (But I'm using 32GB because when I purchase no stock for 16GB)
My usage now is 22.2GB free from total 29.1GB.
You need to think of what kind of thing you want to put in it and estimate it yourself.

Sure you can use iPhone without those plans. Just be careful on your data usage. iOS4+ allows you to completely turn of cellular data.

If your gonna download many apps and games, get the 32gb model
If your just gonna browse the web, call, SMS and watch some YouTube videos and Facebooking etc. 16gb is more than enuf.

Yes. Turn off cellular data and 3G and you're safe.
If you purchase directly from Maxis, the price is RM2,290 for 16GB and RM2,690 for 32GB. DiGi do not sells outright.
If purchase from garage sales/users/traders etc, the price would more expensive a bit, maybe extra RM50~RM150.
Just make sure they are real Maxis or DiGi sets (with warranty card and serial number are same).
Depends on what kind of game you want to play. Some big game like those 3D games will consume more spaces (more than 300MB). While mini/puzzle games may only take up 10~30MB.
iPhone can play movie. But need to convert to iPhone compatible format (mp4).
Or install video player apps such as VLC Player which allows you to play most formats.