We did similar to what you are planning in July – we hired a small country house for a week and invited close family to stay for the wedding weekend and had day guests. We didn't have marquee so can’t help with that.
It's harder than doing it in a hotel because you have to arrange for all the little things to happen, e.g. who will play the wedding ceremony music, or arrange for champagne and canapes to be served. We also wanted our family to enjoy the day and not have to pitch in and 'work'.
We had a fantastic caterer who knew what she was doing as regards organising the day and having the right people in place. For example our 'country house' was quite small so we had to hire in an extra oven to cook the food and fridges/ freezers for drinks etc.
We made a detailed timetable of the day and what needed to happen where and when. The caterer supplied staff which we paid for by the hour. We had a barman, head waitress, 2 waitresses and a washer-upper and the chef.
The caterer provided the wedding breakfast and the evening buffet food. We supplied all snacks, biscuits, tea/ coffee, alcohol and soft drinks. We probably spent about £1,000 on drinks (from Costco, Laithwaites, supermarkets) for 20 drinkers and had loads left over 
. It was a free bar and served by the catering staff. We also set up a tea and coffee station with Nespresso machine which was popular.
Don't forget you will have to transport all the food and drink you provide to the venue. We hired a van! Yes, Majestic etc can deliver but there will be someone at the venue to accept the delivery. Any equipment e.g. oven, fridges, chairs, tables, table linen will need to be delivered and collected. Ditto the wedding cake. Nothing got delivered on the same time or the same day which meant lots of hanging around.