TL;DR: try 2 oz. of crushed med-high roasted coffee per 2.5 gallons of beer for 1 week. Soak it in the beer like dry hops. Report back.
Your question is highly dependent on personal tastes, so maybe if I just tell you mine it'll help you decide what might work best for yours!
Two of the ways that I like to approach a coffee beer are: 1) take a beer that already has coffee-like character and amp it up with actual coffee, and 2) take a beer that isn't coffee-like on its own but has room for a complementary coffee character to be added.
Personally, I tend to make very roasty stouts. This leaves little room for coffee additions before you send the beer over the edge into overly-acrid, burnt-coffee type character. This makes the split batch approach difficult. Because of that, I tend to prefer to add coffee to beers that have room for a bump in coffee character. Moderately-hopped American or English Browns work excellently for letting coffee take the forefront. A chocolate-dominant (not roast-dominant) porter works excellently as well.
I feel that those styles work well, particularly for a split batch approach, because one will be nice and malty/toasty/chocolatey, and the other will have that powerful roasty/coffee character.
Probably the most important piece of information I've taken away from my coffee beers is that the "dry-beaning" method produces far and away the best results for my tastes. The flavor contribution is moderate, but the aroma contribution is powerful - and the fresh-ground coffee bean aroma makes a huge impression before you even take the first sip. It emphasizes whatever even vaguely coffee-type or complementary characters your original beer had in a great way.
I also tend to prefer highly roasted, but not espresso-level roasted coffee beans for this. I drink Peet's French Roast almost every day, but I found it too roasty for beer. Just one step below French Roast (like Major Dickason's, if we stick to Peet's) tends to work well for aroma that most people identify as "coffee" without seeming overly acrid or burnt.
Lastly, think of this process just like dry-hopping. Quantity and contact time play a role. I've liked 4 oz. of lightly crushed coffee beans per 5 gallons of beer for about 1 week of contact. Use a bag so you can pull them out before the beer starts to taste "over-brewed".
Good luck! Please report back with what worked or did not work!
