Guys need some help regarding 3gs, i have never used ios. I am buying 3gs 16gb.
1. How do I check before buying whether it's factory unlocked or not? Also how do I check the baseband and which baseband should I avoid?
2. After jail breaking, can I visit the apple app store from my mobile?
3. When updating/changing firmware can I backup all my installed apps?
4. Is it possible to roll back to ios 4 after trying ios5??
5. If my phone is factory unlocked then I dont have to worry about phone getting locked when upgrading etc right?

1) The model number can be an indicator, and is usually the safest way to determine if it is carrier unlocked. You can check the baseband from the phone itself, it shows up in the Settings.app as the Modem Firmware. A normally current list of unlockable basebands can be found here: iPhone Dev Team - Wikipedia, the free encyclopedia
2) Yes.
3) Yes, through iTunes, like you normally would.
4) Not easily, but this has nothing to do with jailbreaking, Apple imposed this restriction back on the 3GS and has kept making it harder to downgrade since. Research SHSH files and find out if your 3GS has any on file before purchasing. Without them, you can not downgrade.
5) Correct.