You need to be able to follow the ifixit guide. It's close to 100% disassembly.
Is it difficult? I would say no. It took me about 30 minutes to do this in a 2010 21.5", for example. For other people, this is terrifying.
You can buy an appropriate processor on eBay (make sure you stick to the ones that were actually sold, or have the same TDP as ones that were actually sold). Figure on $200-300 for an i7. Probably half that for an i5. Make sure you also buy some thermal compound to put on the heat sink.