ML430 or ML320

Hi,
I am planning to buy used Mercedes ML430 or Ml320 around year 2000. I have few questions regarding that and would really appreciate if someone could help.
First question is what would be gas milage of this vehicle and what would be the milage of same year and same milage for ML320. I don't know if there's any sigifician difference. Mercedes says for ML430 you can get 100kms for 12 liters of gas and for ML320 you can get 100kms for 9.5 liters of gas. I understand this mileage would be under perfect driving condition but wonder how much you get on real road.
Second question is that someone told me to do certain test to check transmission of the vehicle. It goes like this, take the vehicle uphill (facing uphill) and bring it to complete stop. Then put it in drive and take foot off the gas pedal. Now in this case the vehicle should stand still there or move forward but in no case it should role back. Then you do the same thing by moving vehicle facing downhill and put it in reverse gear. It should stand still or move back but in no case should move forward.
I want to know is this fare test to check the transmission and engine.
Third question is was there any big manufactuere's issue regarding 2000 year model vehichle.
Your input on any other important consideration I should be giving while buying used vehicle would be really appreciated

Thanks for your reply in advance