Guns diplomat is totally doable, but you should focus early on getting good with guns. Choose a good background (raised by monks or charlatan's protege for your purposes), get 4 skill points into firearms (req. 15 pe), then do whatever diplomacy stuff you want. Definitely max charisma before beauty, because persuasion and extra followers rules.
As far as MA/TA goes, I wish Arcanum was that cool, but the truth is that TA has very little effect compared to MA.
TA basically only determines "chance of failure" that is: it reduces the effect of the item if your MA is high. 0 TA (neutral, nonmagical) will usually make tech items work just as fine as 100 TA.
MA on the other hand, is the full spectrum, where 100 TA means you get nothing from magic items, and 100 MA means you always get the fullest effect from magic items.
As far as tech items versus magic monsters:
1) there are not really enough of them for it to be a big deal
2) your chance to hit is the main thing affected, which is really determined by weapon skill
As far as magic vs tech monsters, it's basically like magic resistance where high TA monsters have a higher chance to resist a spell entirely.
I suppose if someone really wanted to RP the hell out of a Gunslinging Diplomat (though I think a personable gunslinger would be more fun imo, like a Han Solo type) then one could put points into BE but personally I feel its a bit of overkill and those scarce points could be better spent elsewhere since BE only raises initial reaction
re: TA vs MA
I disagree that there arent many high MA enemies, especially in the early game. The Tarant Sewers alone is chock full of high MA enemies, the Mines have both golems and fire elementals, Ashbury is full of zombies and skeletons, the Maze has high level dread zombies and plant creatures, the site of the dead dragon has magical dragonlings and so on and so on, all of this in the first half (or maybe even first third) of the game
also, although I am admittedly new to AOSAMO I still feel relying solely on rank 5 Firearms and the 16 TA that provides wouldnt be enough to successfully finish the game without giving oneself a headache -unless- (as I stated above) youre okay with your party finishing off those high MA enemies