R134 can and pressure gauge is available at Autozone with instructions as well.
Is this a DIY or stealer has to do it?
My E36 M3 is 5 years old now, never did any repair or maintenence on A/C but last summer i think it did not provide cold air as previous years.

Definitely a DIY. Should only take you 20 mins at the most. I recharged the A/C last week and the air that comes out now is down right frigid.

of R134 did you use, is 1 enough.

I ended up using about one can. The gauge will let you know when you added enough.