You could fix it yourself. I've replaced power, and home buttons on iPhone 4's. Not terrible, but not a walk in the park.
Or, you could get a fully working used iPhone 4 for $50 on Swappa. Seems like that would be the easier option if you must have an iPhone 4. Otherwise, I'd just cut the losses and jailbreak it. You could use Zephyr to replace the home button, it's a pretty cool tweak. Or use activator or assistive touch for power and home since it's on iOS 5.
Assistive touch needs no jailbreak by the way. Just go to the accessibility settings and turn it on.