If I were you I would hire someone else to do it since you have no experience and a deck that big is very, very expensive; $800 sounds about right, maybe even cheap; without seeing the deck my self it's hard to put a price on it. A power washer can be dangerous in the hands of inexperienced people; one inch to close to the wood with the tip of the power washer could ruin the whole deck. Having the right size tip is essential when using a power-washer on a deck. There are to many things that can and will go bad if you walk blindly into it.
Get 3-4 estimates with and without what ever product you choose to use on the deck. Ask the contractor if they will be using a cleansing product and a power washer for preparation before staining; If they don't, then don't hire them. You need to know if the wood is pressure treated or not? If it is, you have to wait 1 year or season before applying anything to the wood.
If you do decide to do it on your own I would get yourself educated on how to do the job right. I believe Homedepot may have classes on deck finishing? You could call a couple of local paint suppliers like Sherwin Williams or Glidden; sometimes they have tutorial dvd's on hand. You may be able to find something on YouTube for a guide to deck staining?
I would definitely use a power washer to clean the deck along with a deck cleansing product. You should have no problem finding a deck cleaning/prep product; just tell the store what your doing and ask for advice on application and quantity needed. As far as products go, as suggested above, Sikkens is a very good product, but it is between $40-60 a quart.. I would use a bug sprayer to apply the product depending on how thick what ever product you go with is. You will need to think about getting at least 1 more person to help you with a deck that size. If the deck is right up against a house you will have a lot of preparation aka: bagging windows or siding that may get stain on it. You can use big pieces of card board to protect siding and what not. I would use a 9" roller cover that is at-least a 1/2-3/4 of an inch thick. and some sort of extension/rolling pole. Also, I use a 3" brush for working the stain/back brushing the stain into the wood. Basically to apply you spray on the product with the bug sprayer and roll on the stain into the floor of the deck. You then will have to brush the stain into the edges of the deck. I usually tape a brush onto the end of a rolling pole and back brush in a standing position. Always make sure you finish what ever you start when you start staining. Always try and maintain a wet edge or the stain may show lap marks. This is why you will need 2 or even 3 people for a big deck. One person is spraying, another is rolling and another is back brushing.
As you can see, if you read all what my blabber, this can be a huge pain in the ass if you don't know what your doing... Good luck.