1. Starting a phD is always scary, especially when many of the scholarships requires you to graduate on time or hit certain criteria e.g. publishing ISI papers. For my case, I have my doubts when I started mine a few years ago (my scholarship has insanely high requirements to be fulfilled), but I knew I have to have a phD if I want to go down the field of Science so I just have to try my best. Turns out to be easier than expected, thanks to great supervisors, good environment and some effort from my part.

In short, how well you perform during your phD depends on:
- Your attitude and dedication. Most of the postgraduates in Science took up phD because if weak reasons (don't want to find jobs; following friends; parents decision etc.), and without the right motivation, its easy to lose sight of your goal and fail.
- Supervisor(s). Supervisors are extremely important! I always ask newcomers to survey on the quality of the supervisors before joining the team. Many supervisors excel at giving people great impression during speeches etc., but they really suck at guiding their own students. Most will say they do not have the time, some just don't care; whereas some will purposely delay your phD so that you can "collect more data" for him/her. Look for supervisors who has the time, and those who care. Most of the time these are younger lecturers. Older ones, on the other hand have better connection and links, and worth being chosen as co-supervisors.
- Environment. Make sure all the infrastructure and equipments are available for the project. Some postgraduate students have to wait up to a year just to get the desired equipment or consumables to run their tests. The people who make up the team (in labs/office etc.) are extremely important too. The number of sabotage cases I've heard in alot of the labs is really sad.
- Luck. In science, many times the results won't be as expected even when you have put in a lot of effort.

2. Most of the time the supervisors are the ones who apply for and are awarded research grants. So you might not be able to propose your own topics because even if you do, there might not be money/ enough money for you to conduct the experiment(s) involved. Try looking up several potential supervisors and see what topics they have, then choose the one you are interested in. Working on a project in which your supervisor has some knowledge and experience has its advantages

3. You can do more with full-time phD, and I think most of the scholarships are catered for full-time postgraduate students too. Postgraduate life can be really awesome, since you have a lot of flexibility in terms of time, and you CAN really do a lot (doesn't have to be phD related) during this time.

4. In the field of Science, phD graduates will likely work as lecturers, continue their research as post-doctorates or research officers. Interestingly, I've noticed many who decided to try an entirely new path after phD e.g. start new business etc. Most people will start off as lecturers because it takes time to build up rep before you can become industrial consultants, professors etc. which opens up a lot more professional career paths.

5. Different field, can't really comment. Sorry