How to prepare:
Do some research. Research the company, the location, nearby places of interest. These topics can provide some good small talk topics in the interview if nothing else. And the more you know the better you will feel walking into the interview. Understand the climate and try to dress appropriately and make sure that you pack an extra set of cloths for the interview and for before and after the interview. When travelling for an interview you never know when something will happen, you could accidentally damage an article of clothing, or you could end up needing to come back a second time the next day. Being prepared for this can lessen the stress in the event it happens.
How to determine if it is worthwhile:
There are 2 considerations that I find most important in this situation. Is it a position I really want, and is it worth the risk financially?
If it is not a position I really want then I am less willing to go above and beyond to land the job. In these cases I want to get as much figured out before I take time off to travel and go through the expense of a trip. Even if the company pays your travel and hotel expenses there is still a cost to the trip. It could be lost wages, or even just time with your friends or family that you do not get. It also may have some benefit like getting a chance to go to someplace you have always wanted to go. I once took an interview specifically because it would be on an 80 something floor of the Willis(then Sears) tower. I had no interest in actually working in down town Chicago and after that interview I knew even if I did it would not be in the 80th floor of a busy building, but the experience is still something I talk about.
If it is a position you really want then you have to weigh the costs that you will incur versus your desire to land the job. Sometimes being upfront and negotiating about issues that are going to come up in the interview ahead of time can work in your favor. When the company goes to the effort of bringing you in for an interview they are predisposed to like you. Any curve-balls you can get out of the way ahead of time will work in your favor. If your disability is obvious and especially if it may require some accommodation communicate that to the person setting up the interview. Providing them with the opportunity to set up the interview so that they can feel comfortable goes a long way.
I also have no problem asking about the salary that a company is targeting when they invite me for an interview. I also have no problem declining the interview if the salary is too low. An interview costs both sides, and if their is no chance that either side is going to be able to accommodate the others salary needs then avoiding the issue in the first place is fine. On the other hand if just making contacts in the company may be useful then do not worry about the salary. The interview gives you a chance to make a contact and present yourself in a good light. Sometimes this in itself is incredibly valuable.