DIY is not a good idea on aircon, as there's no way to insert the correct volume of refrigerant and new lubricant without emptying the system and vacuum purging it first. A specialist will also test the integrity of the system at the same time. Just can't be done properly any other way.
Last time I had mine done, I actually used my local Alfa dealer.
Not only did they recharge the system properly but also gave the air circulation system a freeze clean. When I got back in the car, it was colder than a refrigerator!
Air con works well at both high and low temps...and every where inbetween...cost a bit but done properly.