Whats this pipe?
 

I try to loosen the intake pipe manifold loose but this pipe get in the way? When I loosen pipe to make way, gas hissing out?

Because it a pipe of the air condition.......

I think you are pointing at the fuel cooler. You are losing freon by the hissing noise.

It small hissed around 5 minutes, will effect my A/C? Do I need to top up?

How do I do that?

Your AC isn't going to work anymore now. Thats okay because its winter.

Take it to an A/C shop, have it properly evacuated, and recharged.

You should probably have the A/C shop replace the o-ring seal at that coupling. Usually they will develop a small leak if the joint is disturbed.

Is it expensive to top up? Out of curiosity where do they top up (connectors)? Is it a simple process?

Is there new law where certain refrigerant is now ban, is that impact on my w202 1994 model car?

If I'm not mistaken your car should use R134a, cheaper to refill but still could cost $100-$300 depending where you go.

You don't just "top up". A machine (vacuum pump) sucks everything out, and then some. The a specific amount of refrigerant (usually measured by weight) is added to the system.
As mentioned, your car should use 134 - there should be a label in the engine compartment, confirming this.

Just as well my a/c was not cold enought prior to this, I think it need changing gas anyhow. How often is this a/c check done during service. Mine is now 135,000km & i dont recollect it being done by MB service?

unless you complain about no A/C it's not going to be serviced, there is probably a leak somewhere else in the system.

If you leave it unsealed, you could be letting in moisture, which could damage the compressor, and you will have to replace the dryer.