yes you have to see all Pokemon in the sinnoh dex heres a link to how to findem http://www.serebii.net/platinum/nationaldex.shtml well you can do pretty much whatever you want after you beat them in the battle place because your pretty much done with the game...you can get heatran if you want but you need the national dex and go to stark mountain. at stark mountain you have to go through it with buck as one of those other trainers that follow you. you can beat the battle frontier if you want its actually optional but I suggest doing it for the game to be hours of fun.
Lum berryGrowing at Resort Area and Route 212 when the player first arrives there. (requires HM03 (Surf) at Route 212 )-Pick a Peck of Colors Flower Shop.Sometimes given by the Berry Master.Sometimes awarded after earning in between 3300-3499 points at Pal Park
Persim berryGrowing in Solaceon Town, Pastoria City, and Route 218 when the player first arrives there.Pick a Peck of Colors Flower Shop.Sometimes given by the Berry Master. Sometimes awarded after earning less than 3000 points at Pal Park