Hello Sushant,
I feel your pain, but I think you are worrying too much about site size, etc. First, if you can actually accomplish anything with GoDaddy, please do a white paper. GoDaddy is a SALES organization: "Thank you for calling sales and support, my name is Matt. May I please have your four digit pin number?" Yes, it is a direct quote.
If you do not light a fire under your client, you will regret it long term (a month or two from now). Remember that with an eCommerce site and an entrepreneur it will only grow. I am going to guess that you likely are not a developer of sites and that may be the cause of some of your fear of moving due to the size of the task. Actually, when I look at a 200 page eCommerce site (assuming most are product pages) I do not see anything daunting at all. Many on moz are handling 10,000 pages plus.
Marcus makes a great suggestion in steering you toward WP and I would also say, with a little study, you could likely have a competency in Joomla. Utilizing osCommerce, you can use a WP platform and life is fairly simple. You would just need a web developer (no offense to anyone, but not a designer) with a reasonable track record and someone (you) to supervise and supply SEO direction. Remember, you are not recreating the wheel and by all means keep it simple. Do not go for a full redesign, etc. Keep it essentially the same, move the data. Just get it done.
With a platform and correct architecture, you are just moving data once all is set up.
Yes to all, it is an over simplification, but you cannot let fear stop you from pushing a client. I would rather push the client and lose them than be a party to their downfall. Remember, when they fail it will be on you. I would rather fail making a move than fail standing still.
So, the final push is this: You have this forum for help as a first base; if you push your client and make the move, get a dev with at least a year of experience to assist you with the basics, you can PM me and I will allow one of our developers to answer your questions via phone/Skype/email to assist you with any roadblocks or questions you run into. We will not carry you, but we can certainly move some hurdles. We will provide the instruction but not do the work for you.
So, no excuses, call the client and tell him what he has to do.
Hope this helps,
Robert