Hi there!
It sounds like you've done your research and will provide all the right documents if they require it. You are a genuine tourist and as such are allowed up to 6 months in the UK. Of course, the immigration officer needs to be satisfied you can afford such a visit and intend to leave afterward and it sounds like you have plenty of evidence to satisfy that.
Definitely bring your bank statements and letter from those providing you with accommodation. I'd also bring along a rough itinerary or mention the list of places you want to visit as well. If it makes you feel more comfortable, bring along the letter from the agency. It really depends what officer you get, some of my friends have come and stayed 3-4 months and been questioned a lot and some haven't been questioned hardly at all. In the case of a friend who was heavily questioned, it was mostly his fault as he had no bank statements or proof of funds, no job to return to, no address of where he was staying, and I had booked his flight so it looked dodgy to the immigration officer and rightly so, but it was a genuine tourist visit, he was just unprepared (though he was let in after a period of questioning!)
The immigration officer may also try to establish what your relationship is to your friend, so be honest about that with them about that - many people who end up with these 'horror' stories you mentioned often lie about the relationship (saying they are visiting a friend, when it turns out it is a boyfriend or fiance, for example).
You sound really prepared and as you are expecting the worst, you may be pleasantly surprised. The more documents to bring, the better, they may not ask to see any or they might want to see everything, so its always best to be overprepared! The trip sounds fantastic and I'm sure it will be the trip of a lifetime! Wales is a stunning part of the UK!