#1
I see that the soldering iron I am looking at uses tips that Weller only sells pre-tinned with lead free solder. I will NOT be using lead-free solder, I will be sing plain old 60/40 solder.

Will this cause any problem or unfavourable condition whatsoever for the soldering work or the tip life/effectiveness?

Thanks!!
#2
I'd assume there wouldn't be a problem. Either way, tips are cheap, and you could always sand the tip if the solder doesn't stick. It wouldn't affect later soldering, just the first tinning.
#3
It'll be no problem. Lead free tips are designed to handle the higher melting temperatures of lead free solder. You will quickly wear out a tip meant for leaded solder if you use it with lead-free solder.