The GPU doesn’t care about the CPU, or vice-versa. AMD is probably better value for money right now if you’re intending to replace both CPU and mobo, but Intel will work.
The reason you don’t see AMD CPU + nVidia GPU in premade machines these days has to do with corporate contracts, not interoperability. Before AMD bought out ATI, it wasn’t an uncommon combination.
Um, if your primary use is typing accented letters, why don’t you just set a compose key? The character sequences you need to type are much more intuitive, and you don’t get this type of problem.
In my case, I have scroll lock (the most useless key on the keyboard) set as a compose key. To get “é”, I type scroll lock, then e, then '.
You can set a compose key using setxkbmap, for instance
setxkbmap -option compose:sclk
. (If scroll lock isn’t to your liking, there are a number of other modifier keys that can be used instead—list here, starting around line 810.) You can also specify it permanently using X configuration files, although I don’t know the exact method.