Ya can't get Chespin at the beginning of Rejuvenation, but you CAN get one.
I'd choose Turtwig over any of the starters, cause of EQ. It also has Leech Seed, for recovery, Crunch, for coverage, and Curse, for a tank that can hit pretty hard.
Sadly, it only gets Seed Bomb by breeding, so Razor Leaf is going to be your best physical Grass type move...
But then again, you can get one later on. I think.
EDIT: Chimchar's a nice choice. It gets Flare Blitz, Close Combat, Mach Punch, and Acrobatics. Plus a few other good moves by tutor/breeding like Thunder Punch, but Jan probably wouldn't give us good Move Tutors like Ame wouldn't.