I would go after Ellsbury first. He'll help you the most this season and probably just as much as Kemp (especially if he doesn't stay healthy) the next 2 years. Kemp would be my second choice and Pedroia third. Ryan Braun would be good for the future but he's not going to help you this season.
Get Kemp if you are looking for next year. Get Ellsbury if you are playing for this year.
Honestly, all PEDs aside, Braun is your best target for next year. Although he is out this year, and assuming he plays next year at the level he has for the last several years, he is the second best fantasy player behind only Miggy.
It sounds like you have a pretty slim chance of both gaining/losing ground for this year, so I think I would look to the future and try for Braun. He has the most value for future years in my eyes, and Kemp's recent injuries scare me.