Specific restore question - please help

Looking for sound advice please guys!

So, I jailbroke and unlocked my O2 iPhone 3G years ago and have been using it on Vodafone but it's got the point where the firmware (3.0 - 7A341) is just too out of date. Rather than me upgrading the jailbroken firmware I just want my normal phone again (because iOS4~ pretty much does the things I wanted my jailbroken phone to do in the first place).

Here's the crux. I want to stay on Vodafone but my unjailbroken/unlocked iPhone is locked to O2.
If I do the following, is this the solution to having a normal - up-to-date iPhone 3G?

Restore to factory with this:
http://forums.macrumors.com/showthread.php?t=603129

Use a free O2 pay-as-you-go sim (whack £15 on it) and use this:
https://www.o2.co.uk/apps/help/help?qid=1&q1=2&route=unlocking&case=Handset Unl

Would I then have an off-the-shelf 3G mobile that I can update the latest 3G-compatible firmware through iTunes... so Facebook and other apps shouldn't crash every single time, MMS will work and I can sync without fear of things going wrong???? I no nothing about basebands and haven't tampered with anything...does this affect me?

Thanks.

Any advice would be most appreciated. I

 

Thanks.

It's super slow as it is though.

My question was would the above process work correctly?