Can somebody hacks into a window server through gsm modem?

I'm building a system which uses gsm modem. We receive sms of customer to order products and we send feed back sms to customer. However, I still have some questions while building this system:

1. Is there any way that hacker can enter my system through gsm modem? (I think that maybe gprs and 3G provides them some ways). A friend of mine also build a similar system, his server doesn't connect to LAN however he sometimes connect to server using gprs. After one night, the whole database in his server is deleted. That's why I ask this question.

2. Is there any solution to prevent when some hackers try to spam sms to make the system overloaded?

3. Is there any other risks with my system?

I have a team to build up mobile app to help customer transfer money through sms easily, I wonder if there is any way they can fake our app and make customer transfer money to their account? For example, android app can be automatically updated using android market. If someone tries to build a fake app and upload it to android market, my customer maybe upgrade the fake version. Could something like that happen?

In case these things happen, could you give me some advice how to overcome it?