I'm with TrojanHorse, it all depends on what you're trying to do. Can you provide more details on your current setup and what you're aiming at for improvements?

Regarding difficulty, a lot of it depends on your skills and tools. With the right amount of both, component swaps can often be easy, especially derailers, brakes and shifters. Cassettes/cranks/BBs/headsets can be more difficult because of specialized tools and/or torque specifications. Cantilever brakes can be finicky to adjust. If you don't already know, learn about cable housing for shift vs. brake. Get a good cable cutter. Taping handlebars just takes technique and practice (practice helps everything!). Visit the Park Tools website, there's a lot to learn there.