It is looking promising that I will be starting a GPC campaign very soon. I have not run Pendragon since first edition!

I have 5e, GPC, and 1st edition BoM, BoKL, BoB, BoA and BoU. I don't plan to upgrade to 5.1 rules and 2e of the books I have right away. That being said, are there any aditional books that I should have from the start?

My players are all new to KAP and none of them are Arthurian scholars. They do like the dynastic play aspect though. "Sort of like Game of Thrones".

Is it worth adding the five years at the begining of the campaign with Book of Uther or should I start new players off with the begining of GPC? I don't see a problem with shifting birthdates back five years in character generation and making a few other adjustments for the earlier time period. I am more wondering about the story and game play aspects.

book o battle 2, and the book of warlord. book of entorage is very useful for randomly making wifes,squires, and even knights

Is it worth adding the five years at the begining of the campaign with Book of Uther or should I start new players off with the begining of GPC? I don't see a problem with shifting birthdates back five years in character generation and making a few other adjustments for the earlier time period. I am more wondering about the story and game play aspects.

Depends on what and how you wish to happen in your campaign.

Personally, I think having 5 extra years is good:
1) It gives your first characters some extra time to develop before Excalibur is found and be in their prime when the Anarchy starts, even better to become the Countess' advisors during the Anarchy (or try to claim power for themselves...).
2) It also gives extra time for the character to start a family, so that the next generation is ready to take over with Arthur's ascension.
3) It gives extra 5 years of use from BoU.
4) It allows the PKs to be present all the way from Uther's coronation (and actually, even before) and participate in his all greatest victories (Menevia, Mt. Damen, Lincoln, St. Albans).

Just a few thoughts that were topmost on my mind.

As for what books... Totally depends on your campaign/play style. Warlord is a very good book with lots of stuff inside it, but probably too high level for most campaigns to get the full benefit from it, although there is a lot of adventure/scenario seeds buried in the Castle Appendix and in the Scenario Hooks for Castles in the Anarchy & Baronial Replacement table. The Warlord generator is quite useful for making up random knights, too. Whilst is it one of my favorite books, you don't really NEED it. If you have Book of Manor already, you can make do with it, so while Book of the Estate would be a nice one to add, it is not really a requirement, either.

I think the revised Book of the Entourage is a very good book to help to deal with the supporting characters (squire & wife, but also handmaiden & husband for lady characters). Marriages can be important in all campaigns, and Book of the Entourage has very good random tables for that, if you don't feel like coming up with all the details on your own, and it helps to fix the misconception that heiresses would be the norm as starting PK wives, one of my pet peeves. Book of the Entourage also has rules to increase the skills of the servants and support characters, as well as rules for soldiers, who might become handy as mercenaries if not as retainers. In short, I think BotEnt helps to fill in that NPC part that is mainly handwaved in the basic rulebook. For instance, the (knightly) husband rules can be used for other knights, such as family knights or household/vassal knights of the PKs, or just to track NPKs.

So just my personal order of usability:
1. Book of the Entourage
2. Book of the Warlord (The Grand Tome of lots of good stuff you can adapt to your campaign, and get an overview of whole of Logres. Some overlap with BoU, which also gives you an overview.)
3. Book of the Estate (If you want to focus more on the landholding & especially base building. Overlaps some with both BotW (shares the economics system, of course, so you probably could adapt it from BotW to smaller scale) and Book of the Manor, which you already have.)

Given that you already have BoU & BoM, I would recommend BotEnt even more.

I have not used BoB (1st or 2nd edition) personally, so I am not going to talk about it. Maybe someone else can chime in to tell you if there is a big difference between BoB 1 and BoB 2 (I wouldn't expect there to be).

Personally I dislike BoA, since I am one of those Skill minimalists, and it offends my sensibilities to see UNITS running around with Skills 20+ (e.g. Urban Roman Infantry, with their Armor 20, grr), let alone units like The Old Riders with Lance 30!!! Or all those Hate Briton 20, base skill 20 Saxon troops... Grr. Again, I have not used it, only leafed through it.

BoA & BoKL both make a mistake of giving Cuirbouli 8 pts of armor, when it should be 6 points, but that is easier to fix.