Put .m4a at the end of the search. eg: artist - title .m4a

I don't recommend converting mp3 to m4a. This will result in a loss of quality. And not good for others if you are resharing these files.

I'm not sure why you're having problems with mp3's, but only mp3Pro are mp3's that iTunes won't recognise. But they're not so common & are usually only used on low bitrate converting. I presume you've seen this: How to transfer music to my iPod (& iTunes)