Hi merchant9,
I can fully appreciate your concerns as purchasing a car is a very significant investment, more so in Malaysia due to the high cost. The problem with asking for opinions on any model of car is the same wherever you go: ask ten people their opinion, and you will likely end up with 11 different answers

My decision to go for the Prius C was based on several factors:
1. It's a Toyota, and they have a decent reputation.
2. It's (at the moment, anyway) CBU, so hopefully it is better assembled.
3. Were it not for the government incentive, this is essentially a RM150K+ car (in Malaysia, anyway.) Malaysia may be quite unique at the moment in that we're the only country where a hybrid is cheaper than the non-hybrid model.
4. All of the glowing reports from the many happy owners around the world (and yes, I read all the negative ones as well.)
5. All of the performance figures were acceptable to me.
6. 7 airbags! Woo!
As for your questions on upgrades, it is important to remember the Prius C is a relatively new car, and as such the availability of upgrades and bolt-on goodies are a bit limited locally. As it is the second highest selling car in Japan, the market there is booming with accessories, it will just take a little time to reach our shores. I am more than happy to wait.
To answer your questions as best as I can:
1. Tinting options are the same as any car in Malaysia, from utter rubbish to ohmygodthatsexpensive.
2. I've only seen one company advertising leather interior upgrades, and at the time they did not list the Prius C.
3. I've seen Prius C body kits advertised between RM1600 to over 6K, but cannot comment on quality of the product.
4. Many report that the OEM DVD-AVN option from Toyota is problematic (it is the same model used on several different Toyota models, not specific to the Prius C) and I feel it is overpriced.
As far as warranty is concerned, according to the sales assistant I spoke with, just about any change/addition can void the warranty. Changing the rims would void the suspension warranty, installing window visors would void the door warranty... You get the idea. It seems that the general practice is to either have them installed by the service centre or to remove them when servicing the car or claiming any warranty. Naturally that isn't always going to be possible/practical, and some things don't effect the warranty (I doubt adding floor mats is going to effect the chassis warranty!)
Anyway, enough rambling. I hope you will be happy with whatever car you choose. Good luck!