Hi. I live in England so the same temperate climate as England. Guinea pigs are hardy animals.
In the summer the guinea pigs can stay out at all times as long as they have the covered box for sleeping with planty of bedding and a covered area for eating and some play in the rain. Make absolutely sure the sleeping are lets no water in, this may mean covering it with some plastic sheeting. It is best to confine them to the covered areas in the rain and to keep them off any wet grass.
In the Autumn and spring more warmth needs to be provided. You can achive this by moving the enclosure into a porch or shed or by taking further measures to insulate the sleeping and eating areas. Maybe fit some king of a thermometer to check the temperatures at different times of the day to check it is suitable.
In the winter they really should be in a warm shed or something similar. If the temperature reaches freezing overnight then it is best to have a small indoor cage to use to get them out of the cold.
The more guinea pigs you have the more combined body warmth so with six as long as they get on well they should be fine.
Outdoor run problems include:
Making sure you protect the wood from the weather but with a varnish that will not hurt the pigs if injested.
Foxes and other preditors; you may need something stronger than chicken wire and you must make sure they are confined in the safest cpvered part at night when the preditors prowl.
I find the hinges and bolts soon wear out and need replacing as they get so much use.
Don't worry about foundations or sinking it into the ground - guinea pigs don't dig.
They will eat and mess the grass quicker than you imagine and the run will kill the grass where it makes contact so move it often.