Questions about Thermostat housing O-ring and A/C

[keystone]

While installing a new fan clutch, I am also updating the thermostat housing with metal one. Mine still has plastic one. I am also changing the thermostat. I took the old one, Wahler and am putting Behr. I was ordering O-ring. There are two kinds for the thermostat: round and flat one (there is also weird shaped orange one, but it goes to the other whole). I got all, but when I was ordering, the online store, I ordered the parts, stated that I need round one for the metal and flat for the old plastic. I wanted to make sure that I will put the round one with this updated one. Please tell me if you did this job so that I put the right one.
The second question is related to A/C. I have not charged or anything for the last 6 years. It seems relatively okay, blows cold, but not efficient as before. I thought I should charge it. I did a search, but could not find a message and there does not seem to any write up for DIY. Anyone tried to get the kit from auto part stores (seem like interdynmics recharge kit is everywhere, including even in walmart). Directions and everything seems easy to follow. But I wanted get your opinion on this as well. Would it be okay to just get one those interdynamic recharge kit and do it. Or should I not mess with that and take it to a mechanic.
Thnaks for your time. I have 95 525i, auto.

[Tiger]

For AC, it is simple... you can get a can with a gauge built in... and hook it up... You want it to be around 28 PSI. If it still doesn't feel icy cold as it should be, then you would have to find a pro... they will evacuate your system completely and reinject proper amount of refrigerant.

The reason sometime it is not a good cooling power is because there is air in the system.

[keystone]

For AC, it is simple... you can get a can with a gauge built in... and hook it up... You want it to be around 28 PSI. If it still doesn't feel icy cold as it should be, then you would have to find a pro... they will evacuate your system completely and reinject proper amount of refrigerant.

The reason sometime it is not a good cooling power is because there is air in the system.

Thanks Tiger.
Now I need someone with thermostat experience to help me with the first part of my question. As soon as I hear from someone, I will start the job as everything is on the floor in the garage for installation.
Thnaks

[Tiger]

Just do it as recommended. Metal wasn't available last I did my thermostat... very easy job.