Assuming all the bits work and there are no leaks.
To do it properly.
New drier about 50 quid
New O-rings and ester oil about 20 quid.
Conversion fittings about 20 quid.
Regas.
My system is pretty powerful and you can get 1C out of the vents, but thats because the cycling switch on mine doesn't work properly and also my condensor was leaking, so the shop that did the initial work put a new condensor in it which is a 'parallel flow' type and much more efficient than the original type. I reckon it would probably work OK with the old one.
According to the green book, 5-8C is the cut out temperature to stop the evaporator freezing.
Its pretty powerful, but you won't ever feel really cold because you've got all that heat coming in through the glass windows and hot dashboard.