1) Buckets are good for a long long time. yes, they may get stained and always have the fermenting smell. If it really bothers you, let it sit outside for a period of time. The UV sun rays will take the smell away. Otherwise just soak in oxy between uses and make sure there is no gunk. The stain and smell will not affect your beers.
2) I have only seen the 3 gallon carboys
3) Lots of people use refractometers, I would go that route for your small batch size.