Right now I'm using a Hunter Pistol but I unlocked a Repeating Pistol, should I buy it?
Repeating Pistol is one of the most damaging firearms in the game, second only in Cannon in terms of raw damage, eclipsing even Evelyn provided it and Evelyn are in the same level +10 and the Bloodtinge stat is similar (say, 50). It is very good to use against enemies with obvious weak points such as Cleric Beast, Darkbeast Paarl, Amygdala, Ebrietas, etc, but for human enemies/bosses Evelyn will generally serve you better due to parrying mechanics.
For pure damage though, Repeating Pistol is the highest. Well, again, except for Cannon, but for sustained damage Repeating Pistol is better than Cannon, in my opinion (powered by Bone Marrow Ash obviously) since you can only fire Cannon twice making carrying 10 Bone Marrow Ash a redundant thing to do.
I personally took Evelyn for all-purpose stuff, since it can also cause considerable damage to the likes of Amygdala/Ebrietas and it also serves well against humanoid bosses for parries.
However if you use Reiterpallasch, then I suggest you take Repeating Pistol, since Reiterpallasch' shots from its gun form eclipse Evelyn in terms of raw damage because most likely you'll be slotting it with various gems if you use it as your main weapon and it serves well for parrying, and for really high damage you can use Repeating Pistol with Bone Marrow Ash.
So I met Eileen for the first time but I didn't realise she was someone you could talk to so I tried stealth attacking from the back and got my ass handed to me.
After looking up a little info on her, I tried talking talking the second time I meet her but suddenly she just attacked me on sight. Is that supposed to happen? Did I just ruin anything related to her for the rest of the game by attacking her the previous time?
Yeah, basically. You'll miss out various gestures and a particularly useful Oath Rune, but everything else you can still get, including her weapons/outfits. At least not until NG+