You're almost certainly better off spending that money on going touring.
I'd go so far as to say that until you have done enough touring to have a list of things that you want from your touring bike that you can't get from any production bike, don't bother thinking about it. Well, unless your other hobby is spending money, in which case I suggest buying one from each custom builder and seeing how they compare.
The main advantage of a custom frame is that it's built to fit you and has exactly what you want on it. Long term tourists often develop quite specific requirements and no mass-produced bike will satisfy them. Even simple things like built-in racks are depressingly rare in mass-market touring bikes, let alone more specific things like eyelets for tying things to. But again, unless you already have that list of things you're wasting your money getting a custom bike.
The main risk is that you spend a lot of money then decide that it wasn't what you wanted after all. This is especially the case with people who haven't done a lot of touring, and I know at least one builder who refuses commissions from novices for this reason. Every unhappy customer they've had has been a newbie to whatever they wanted the bike for. The solution is to put them on a cheap bike and wait a year.
With S&S couplers you're likely to need those added after you buy a mass-market bike, which is an opportunity for you to get the framebuilder who does the work to add any little extras you want. It'll very likely be cheaper to buy a new mass-produced bike and have the couplers added than get a customer bike. But it's likely to be cheaper to pay a freight company to ship the bike around the world a couple of times than to get the couplers fitted.
As Alan says, you can tour on any bike. I've done a 5Mm trip through northern Australia with people who rode BSOs (bicycle shaped objects), one of whom did the last couple of megametres with no balls in their bottom bracket (and hence, no front derailleur). They didn't enjoy the riding as much as they could have, but they did it.
As far as comfort goes, you will very likely be more comfortable on the cheapest recumbent you can find than the most expensive upright bike.