main battery!

when i bought my 1987 635CSi (uk spec) it had no battery. the po had disposed of it when it went flat. i took along an old spare one i happened to have. however it was from a porsche 924 and it's just not up to the job to cranking a big six. i need to get to my extra cheap motor factors before 2pm (it's 11:15am now) if i want to buy a new battery today, the problem is i don't know what capacity (Ah) it should have!

i have no handbook for the car and all the google / bmw web site searches i do only return info in the si batteries... :(

can anyone help??

crisis averted, i found out the battery i needed (bosch 7e) and bought one. :)