I recently built my first and only 5.56 AR and it was a great experience. Id love to build a .22 upper if I can do it rather cheaply so I have a few questions.
First, what differences are there between a 5.56 upper and .22? Like for buying parts would I buy a conversion kit + .22 barrel and the rest would be the same?
Any recommended barrels or bcgs?
And will it work fine with a standard rifle buffer (I have the PSA MOE kit)?

Im familiar with how to build a 5.56 upper I just dont know how different a .22 is. Is it the exact same just slide in a different BCG and barrel?

and can a .223/5.56 flash hider be used? Id like to build a .22 replica of my 5.56 and im not sure if there is a .22 version of my flash hider

See this page of the thread through the end. I started building building and shooting 22 uppers from this point on.
You'll get some ideas of what can be done. The builds are scattered throughout the thread and the write ups should
answer all of your questions,

Dave S

http://www.ar15.com/forums/t_3_15/441890_22_Conversions___Dedicated_22_s__22_Builds___Range_Reports__Ammo__Tips__Tricks__Insight_and_More_.html&page=3

We don't know what your current AR looks like so we can't say. But, why not go over to the CMMG website and look at what is offered. If you are starting from scratch you may find it is more cost effective to just buy one of their dedicated uppers that is already done. You can change the hand guard if need be to match your other rifle.

But, do the math. Price out each part separately. You are going to have to buy a lot of parts starting from nothing. I have done it both ways and sometimes its cheaper to just get the package if there is something close to what you want.