You'll be a lot better off with your stuff in panniers.
Assuming 350-400 miles in 5-7 days, you'll be doing an average of between 50 and 80 miles a day. You should try to ride that distance in a day sometime before your trip to see what it's like.
Unless you intend to actually tour off-road, get tyres that are suited for touring on tarmac.
Practical repair experience is kind of important. Try doing different repairs on your bike as it is now, even if it isn't broken. Then you'll know what to do if something breaks.