I will try to give you a start.
First a question to you: Are you allowed to do this under the terms of service of your isp?
1. Do i need to upgrade my ADSL plan?
Unless you are planning on a very low trafic site, yes. I am doing something similar and am using a fractional T1 line with 768/768
2. Do i need to setup DNS Server or How can i point those 10 web site from external to my server?
Depending on the registrar, you may be able to point the various domain names directly to your IP. If that is not possible, there are a few DNS services available that charge a small fee for the number of domains you are refering to. Look at dnsmadeeasy.com, orzoneedit.com
3. What sort of H/W server i need?
It depends a little on what you can afford. I am using 512MB ram,a Athlon XP-2600+, and 2-120GB ide hard drives in raid configuration.
4. Can i setup send/view Web Statics to my customers
There are ways with AWstats, but get your feet wet with SME first. You can add that part later
AS I said, these are starting points, and there may be better ways of doing this. I am far from an expert. This is a great distro because it is already screwed down pretty well as far as security and stability are concerned.
You will want to add a few other addons to allow virtual email, virus scanning etc.
Look over the contribs and forums and you will find a load of information on addons.
Bob