I'm planning to buy second hand iphone 3. Can I still upgrade it to iOS 6?

Do I need something like receipt from the original owner or any other document or proof purchase in order to upgrade my iOS or for any other purposes?
What are the disadvantages that you still think of iphone 3?
For how much I can buy it?
advantages of iphone 3 vs 3g

There was no iPhone 3, there was the iPhone 3G (the second iPhone), and the iPhone 3GS (the third iPhone and the uprated iPhone 3G).

There's no iPhone 3. 3G or 3GS. No you wouldn't need those things to update it. Just an internet connection & iTunes.

The iPhone 3G only updates to 4.2.1
The iPhone 3GS only updates to 5.1.1

Price of course depends on who & what condition & where.