1. Premade vs Custom Made for 8ftx8ft?
Premade since it is only 8x8. Premade quality is visible while custom made is not.

2. Melamine wood no good at all? Malaysia weather is it worst for it.
Melamine is more than enough for wardrobe which typically at dry area

3. Sliding door confirm can't close tight within 3-5 years?
Depending on the mechanism you spend. Do not expect consistent quality over time when you only pay for cheap price

4. Any furniture store suggestion?
Mixbox, they are selling in premium

Did TS ask for L shape? i am reading 8x8 as L x H which is normally how the industry quote 

Swing door is as cheap as 350 per foot run, most of the provider are giving hafele hinges already. It shouldn't be a problem in long run

Sliding door is around 430 per foot run, sliding mechanism is the concern because those premium sliding mechanism is not common in MY and pretty expensive.

You are right, mixbox only do straight design; or i should say their design is pretty boring but quality is assured.