Before you buy, try stuffing all your gear into it. You might need a bigger one than you think. You don't need to buy all the gear the first time out. Although good footwear and clothing is essential, many outdoor shops offer a hire service for both children and adults. Conditions can change very rapidly in mountainous areas during the day, and the weather on the fell tops can be very different from down in the valley.
Good visibility can also change suddenly to thick mist.
Although there are several telephone services providing advice of weather conditions in the mountains, many are premium rate numbers which can cost a fortune from a phone box. But to be enjoyable you do need suitable gear. Keeping dry, warm and well fed are the priorities. The tent must be able to stand up to bad weather. We once spent a wild night in a tent which was being repeatedly flattened by the wind as the poles bent under the onslaught of every gust.