Learning spanish

Peru and Columbia are the closest to 'Pure' castillano, IMO.
But everyone will understand you, in general.
Now Quechua, that's a different story...
But it is doubtful that you will find yourself in a 'Quechua only' situation.
There are lots of Peruvian Idiomas, but they are easy to suss out.

Thanks, if its just idioms and a few words that are different, like lift/elevator in british english and american english its no problem. I just more worried that it had shifted from spanish spanish to a new kind of spanish with new rules and vocabulary.

Believe me, you should have no problem. It would be similar to an English person coming to the US. They could still talk with out a problem.

I did a Spanish intensive course for 2 months before visiting South America, and had also learnt Castilian Spanish years ago before a visit to Spain. They understood me perfectly, even though I was still using the "th" sound instead of "s". Had a ball! It's great to chat to the locals.

No problema. Peru, Columbia and Dominican Rep are the closest to Castilian IMO.... furthest from Castilian would be Mexican, Puerto Rico, Cuba, Costa Rica, Guatemala rest of cent america... even in Mexico you would be understood apart from idiom.

The difference between peruvian spanish and european spanish is not af great as the difference between american english and british english.
With that said you will still need some time to pick up on the accent, slang...etc. By the way latin american spanish is basicly the same thruout the americas.
Only accents and modalities will differ between regions.