Bought a friend's 3 year old iPhone 3G for €75 some time ago. It worked perfectly, except for the fact that when used actively, I had to recharge it 2-3 times a day. Although I had no experience repairing electronic devices, the repair guide and low price for the replacement battery gave me enough confidence to give it a go.
It was kind of scary but mostly just exiting to open an iPhone for the first time. The repair guide was comprehensive and really guided me past every step of the process. The tools that iFixit shipped with my new battery worked great and were all that I needed to successfully complete the replacement.
Don't be scared by the tiny, vulnerable electronics in your device. Get accustomed to seeing these by studying the repair guide. Just take your time and you'll see that anyone can perform these repairs and save themselves €500 for a new iPhone when their old one still works fine after investing €15 and half an hour of work!