Hi Changfeng, for of all, best of luck with your business. Like all businesses, you need to stay organized and a Mendix sandbox is certainly a good option. To the best of my knowledge:
3: initial databases are about 50MB or so. 500MB will indeed take you a long way. And as long as you are wise with the 1G for files, pdf's, photo's and so on, you will be good there too.
Any other risks:
- Mendix deciding to stop servicing the sandbox. Not likely, but it is a risk.
- security, you are on the Mendix cloud, which is secure and so will be your software, but who is monitoring your app for any security mistakes.
- backups. These are kept for two weeks only, which is already great for a free app, but might not be enough for you. If so, download them regularly and every once in awhile test if are able to restore them.