The year 800 and Charlemagne held great significance to Hitler, because Charlemagne was held in high regards by Hitler for his military prowess and his work "civilizing the slavs", but more importantly Charlemagne was the founder of the first truly German state, the Holy Roman Empire. Here is the relevant quote from Mein kampf:

The word REICH, which is a German form of the Latin word REGNUM, does not mean Kingdom or Empire or Republic. It is a sort of basic word that may apply to any form of Constitution. Perhaps our word, Realm, would be the best translation, though the word Empire can be used when the REICH was actually an Empire. The forerunner of the first German Empire was the Holy Roman Empire which Charlemagne founded in A.D. 800. Charlemagne was King of the Franks, a group of Germanic tribes that subsequently became Romanized.

Now since Hitler was a believer in the idea that Germany could trace its roots directly back to the Holy Roman Empire. So to him, the founder of the first German Empire being crowned by the church was the first time that a sovereign German state had been made subordinate to the Church. Hitler didn't hate religion so much as he did religion that tried to interfere in the state. In Hitler's mind the year 800 is when Christianity stopped just being a "religion" and actually became more powerful than the German state. Hitler felt that the Catholic Church from that moment on had done nothing but try and subvert and undermine the German Government and German people. Hitler talked in private about how he felt that the mixture of Church and state had led to the problems in Spain and he offered up this choice little quote that shed's more light on the issue:

It is with such semblance of humility that the Church has always wormed its way into power and succeeded in winning its way by flattery into the good graces of the German Emperors, from Charlemagne onwards. It is the same technique as that employed by sophisticated women, who at first exude charm in order to gain a man's confidence, and then gradually tighten the strings, until they hold them so firmly that the man dances like a puppet to their whims. With a little diplomatic savoir faire such women manage even to persuade their husbands —exactly as in the case of the Church and the German Emperors —that it is they who rule the roost, and this in spite of the nosering on which they are so obviously being led !

Here is another relevant quote:

The development of relations between State and Church affords a very instructive example of how the carelessness of a single statesman can have after-effects which last for centuries. When Charlemagne was kneeling at prayer in St. Peter's, Rome, at Christmas in the year 800, the Pope, giving him no time to work out the possible effects of so symbolic an action, suddenly bent down and presto ! popped a golden crown on his head ! By permitting it, the Emperor delivered himself and his successors into the hands of a power which subjected the German Government and the German people to five hundred years of martyrdom.

You have your answer within your own question, but you don't see it.

'strange and foreign Christian faiths' meaning the Christian faith is strange and foreign to Germans. The plan you mention further elaborates by stating that Mein Kampf must be on the altar, and in articles 18 and 30 states that crosses and bibles should be banned. It's not a theological issue.

800 was chosen because it marked to some a watershed in Christianization of German lands (under a Germanic Charlemagne). It's propagandistic, not historical.