for what it's worth I have a 16 year old? REI Randonee. I used it to do Portland to SF 15 years ago, and plan to do the TA with it next summer. There were a couple of hills where the 25 inch low wasn't enough, and I swapped the 26 granny ring for a 24 a couple years back to give a 23. That will be borderline for the TA I think but OK. And of course some components have worn out and have been replaced over the years, but it's a good steel frame. It sits outside at work about 4 days a week, and the paint is cooked from the AZ sun, but it still rides just fine.
Guess the point is, get a good solid mid range bike and it will last you for years. Expensive frequently means ultra light which means fragile. Cheap means poor quality which means quick wear out. In the case of mine, the only significant "non-wear" item to break was the STI brake/shift levers. Replaced with straight brake levers and down tube shifters and never looked back.
Only thing I need do before the TA is grease, new cassette, chain, tires (worn from commuting), and seat (16 years old, sun cooked).
Now I did spend a bunch on a new tent, sleeping bag, pad, new Panniers....