Diablo Game Questions Getting RoS?

I got into Diablo 3 lately and I'm really loving the game. I'm currently playing in the Free trial, and I'm planning on buying the game. I found out about the RoS expansion and from what I gathered it seems pretty much a must-have for end game. I was wondering if it's okay to play the regular game until the end game and then getting RoS? Does RoS affect the pre-endgame gameplay? Also, any tips for buying the game for a good price? I understood that buying from Battle.net can be expensive...

Thanks!