Don't forget as well as the Marquee you will need tables, chairs, crockery etc., all to be hired. You will also need to think about caterers (if you are not cooking yourself), decor etc., Also if you have a big party, you need to think about loos as well.
It's a couple of years since I priced this up for a similar event, but I do recall that between everything it worked out just as good value to use a local hotel; we had a sit-down meal, drinks/cocktails on arrival, & wine on the table with the meal etc., and a lot less work overall.