mehran is good for you as your first car, newer is better then old.
in documents as you are in karachi
i)
1= Get CPLC n ACLC clearance first
2= Get car Book + return file + number plats + transfer letter
3= Make sale dead pay Car price
ii)
PW + olx is better, n pay how you feel comfortable
mostly ppl prefer cash then pay order then check
n yes you have to pay all amount before transfer
iii)
you need appearance of origional owner in excise + car doc ,,, if owner not available, you have to pay 5-6 extra in excise as rishwat
i have lots of experience in car dealing, i suggest you to trust on your Friend office colleague etc instant of some one paid man, inhen sirf apny 5-10 hazar se garz hota hai, kuch iman dar b hoty hen but wo 1% b nai hen, mainy aysy logon ko dakha hai jb kisi ap jasy bandy k sath gari dakhny aty hen to chupky se kan main kahty hen "bhai 5 hazar ager mujy dain to gari bikwa data hu" ok ka ishara do to aysi aysi khobiyan nikalty hen hamari gari main k un ka hamen b pata nai hota.