How much refrigerant will fit into the system if it's empty?

Can anyone recommend a particular kit that includes some sort of stop leak? I'm assuming the system is empty due to a leak.

'98 Mystique Zetec w/ auto trans 103,900 miles Kenwood KDC-MPV7017 CD/MP3 HU Kenwood KFC-6889ie 6x8" 3-way speakers '96 Mystique Zetec ATX 108,000 miles