I would caution anyone against spending too much money on a first touring bike when they don't have much experience (any?) If this doesn't apply to you, my apologies. Interpret this as a general caution to a generic newbie. When I say too much money, I'm talking about over $2000. Something like custom titanium with S&S couplers.
I would also recommend spending a good amount of money for a good first touring bike. Sound like a dichotomy? Lemme 'splain.
Having the right rig is very important when you're carrying a big load on a long tour. You need comfort, handling, the ability to climb hills, the ability to stop, the ability to attach racks, maybe three water bottles, etc. Some people like to tour on a bike with an internal-gear rear hub, like a Rohloff. Some people like butterfly bars, while some like drops. Etc. What is the perfect bike for you may be a dog in my book. It's personal.
So, if you've got the money and you're never toured, my suggestion would be to buy a good, solid, reputable, mainstream tourer. Something "safe." Like a Surly LHT, a Trek 520, a Cannondale tourer, etc. $1000-1500 will get a nice bike, and there are even cheaper options (Windsor, etc.) You may fall in love, but you may not. Either way, you'd have a bike that would get you through your first tour in good shape, and get you thinking. "What do I want for my next tourer?"
One nice thing about touring bikes is that they hold their value. If you buy a "mainstream" one, you can sell it used for a good price. So if you decide the bike you have is no longer suitable and you want to move up, you'll be able to recoup much of the original investment. (60%? 70%? More?)
I always carry a notebook on tours and take notes about things like:
- What do I wish I had brought?
- What did I bring that I ended up not using?
- What would I change about my rig before the next tour?
This would be very valuable when designing that custom titanium beauty you may want.
All of this is based on the assumption that one has the money for this. If not, there are tons of threads about finding a bike to tour on without spending too much.