You need to do a LOT of research/homework before you get much further into this mess as it can be daunting even if you know exactly what you want.
First and foremost...decide on which platform you want to build on. The primary options are Armalite pattern and DPMS/KAC pattern receivers/barrel extensions/bolts/carriers. Both are excellent platforms to build on but differ primarily in their magazines and some parts compatibility with their AR-15 little brothers. The DPMS/KAC pattern rifles have really taken off/over the last several years based primarily on the sheer multitude of different mfgs making uppers/lowers/aftermarket parts/accessories for them, and largely (thanks BARFCOM) to the introduction of magazines by Magpul that work on the DPMS-pattern rifles whereas with Armalite, the only company making mags for their rifles (the AR-10B rifles anyway) is Armalite. Both are excellent platforms to build all manner of large-frame ARs from...but only you can decide which is better for you and which particular mfg under which category makes the receiver with the features you have decided you want/need.
As for caliber selection...while .308/7.62x51 certainly passes the Two Dot test, is relatively cheap depending on what you feed it, and is capable of outstanding accuracy and performance within certain limitations, it may or may not be the best for you depending on your end goals with your rifle. Knowing more about what, specifically, you want/need out of the rifle in terms of max range, targets you'll be shooting (or game you'll be hunting with it), whether or not you reload (or plan on it at some point), etc., etc. would help this discussion along as there are dozens of different chamberings available for the large-frame ARs and they each have their own pros/cons.
Similarly, depending on which caliber you select and what your ends will be with the rifle should point you in the direction you'll want to go in terms of the barrel you select. While I know you said you have "a friend" who will make you a barrel, depending on how much money you'll pay said friend to make it for you, whether he has ever done an AR barrel and can properly set it up for you, etc. may be worth considering just buying a high-end, off-the-shelf barrel (there are MANY quality tubes out there for the large-frame ARs). The other thing to consider is sourcing a correct barrel extension for your build based on which platform (Armalite or DPMS) you select. Finding Armalite bbl extensions can be a PITA at times to put it mildly.
As for other "parts" to use...that just opens up a whole can of questions. First decide on how you want to proceed with the above questions...then focus on how best to achieve your goals from there. You'll need to select a specific pattern receiver before you start picking parts because for some things, you can NOT cross platforms in terms of compatibility. Thus, you'll need things like handguards/rails listed as being for DPMS (couple different generations of receiver/rail height) or Armalite and you can't use one on the other.
The other thing to consider when building any AR, whether an AR-15 or a large frame AR, is that it isn't something that can be done with your average hand tools in your basement/workshop. You'll need some specialty tools to properly put one together and they can get expensive if this will be a "one-time thing" for you in terms of building it. This is one of the reasons that many people outsource their builds or otherwise buy a factory rifle that is already setup in the exact (or close) configuration that they want. Just something to consider.
Anyway...food for thought for you before you get too far down this path of trying to figure out what is best for you. Once you decide on all that...we can probably better point you in the direction that will get you to where you want to be.