It isn't that hard, But it might be a bit overwhelming if you haven't done something like it before. I would recommend setting aside a couple of evenings for the job and taking your time. Start by stripping everything down, the build your bike back up slowly with the new parts.

It might be easier to pay your LBS for the job, but if you do it yourself, you are going to be well equipped to handle most issues and do any maintenance that may arise in the future.

Also, you can always bring the bike in to your LBS for the final setup and adjustment.

Here are some things you might want to keep in mind/check:

• if you are replacing brake/shift levers, you will need to rewrap the bar tape. It might be possible to reuse the old tape, but it might get destroyed in the process. (In my opinion, just get new bar tape. It's not that expensive and directly impacts the (literal) feel of the bike
• different types of bottom brackets need different tools. Check that you have the correct one before taking everything apart.