Do I need to get a travel visa in Washington DC or I can avail the 5 day Visa on arrival?

Marvin, you can rely on obtaining a 5 day Shenzhen visa on arrival and it will be granted to you. Anyway, if I were you and especially because you still have enough time before your planned travel, I would opt for a prearranged 10 year Chinese multiple entry visa. When you take into account that the price for a Shenzhen VOA for US citizens is 956 CNY, then you will perfectly understand why I'm suggesting you to obtain a Chinese visa in advance.

When we talk about the prices, then obtaining of a Chinese visa could save you quite an amount on your air fare. For example, if you fly to Shenzhen or Guangzhou that are quite close to Hong Kong, you can usually get more affordable price than flying directly to Hong Kong. OK, now when you know some of the suggestions, it will be up to you to decide :)

In the event that i will apply for a for a China Visa, part of the item in the application is a letter of invitation which I don't have. In what way can i obtain this requirement?

You, don't need a letter of invitation. Your flight tickets and a short itinerary with a couple of booked accommodations will be more than enough.

thanks so much for the info, will be working on my visa then..keep you posted