I think you'd be able to get med mal insurance. I don't know if you'd pay more or not. I suspect that most insurers wouldn't have a specific program for a travel clinic and would lump you in with either primary care of urgent care. In reality your liability should be low. You would be presumably following CDC guidelines so you could fall back on that if there's a problem. Your main risks will be from people who have reactons to the meds and people who get sick despite your efforts. Either of these can be helped by good info/disclaimers.
As for profitability your main issue will be volume. You may or may not be incompetition with other docs and PMDs for this in the area you choose. I suggest picking an area near a busy intl airport. Plan on being available 24/7 while you get the practice going and cater to people who have to make unplanned trips. You could charge accordingly. Make your practice known to all the local travel agents, airlines. Make sure you have a website that will get indexed by the big search engines so that people who buy tickets on line and then search for travel med will find you.
All this is good advice. You might also look at other "cash" for service care. One is DOT and insurance physicals. These are usually pretty good as well. Organized medicine and the ABMS has done its very best to finish off the ole fashioned GP, but if you are willing to forego the insurance panel issues and work on a cash basis, with low overhead, you might be able to make a go of it.
As others have stated, the biggie is the capital resources to get the practice going, covering the initial overheads which include rent/utils/taxes/equipment/insurances (property/injury/medmal) and putting bread on the table. In a startup practice, you may be able to get by without any staff, if you are willing to be the drawer of blood, runner of lab tests and follower up of scheduling a phone calls. This gets old in a hurry, but it's a way to start.
I've run the numbers in the past based on the typical hmo copays in the area. Here's the basic numbers from a nearby clinic (my best guess since I don't have them at my fingertips).
Office visit (int/prob focused new patient) $125
Office visit (int/prob focused return patient) $70
DoT physical $100
Plus you can bill for other services as well.
If you work in a big MC area and are not on their panel life becomes a tad more interesting. Depending on if they are open panel or not you might or might not be considred a primary care doc for the purpose of referals for secondary care, at least in some states. As always your mileage may vary.
But I've often thought of this. If a GP doc not on an insurance panel decided to charge a typical HMO/Insurance co-pay and built a busy practice, of say 9 hours a day with on the average 3 patients/hour, that would bring in 9x3x$25-$50 (in our area that's what copays are these days) or around $1000/day or $20,000 / month. Subtract about $5000 of that for overheads and that leaves you with $15,000/month. A nurse/assistant costs you $5,000 which leaves you with $10,000 x 11 months or $110,000, just from charging the co-pay amount, assuming you can collect it all. You don't have a medical office biller. You don't have insurance collections chasers and those are big expenses. You or your nurse answer the phone, and it's a real 'mom and pop' operation.
On the other hand, if you were able to bill *and* collect on the "list" price, $125 ($50 copay, $75 from insurance companies, less the negotiated discounts, of course), you might make an extra Jefferson or two an hour so we add $10k/month. So, you make a lot more, but some copays won't happen, some insurance companies will make you wait months, and you now need someone to argue with the insurance companies who don't have you on their panel, at a cost of an additional $5k in wages, extra telephones, faxes, computers etc, so you net out $5k/month or $60k/year.
Capital costs up front are not going to be free, of course. You need supplies, instruments and such, but it is doable. If you go the Medicare route, that opens up some too, but check carefully the costs of doing business with the government. It ain't cheap! Plus you get a bit more for doing office procedures, too.
As for folks with insurance, you can offer to help them with insurance forms, but do not get sucked into the direct pay/direct bill routine, in my opinion. This opens the door for insurance companies to a.) exclude you from the panel, b.) control the finances of your practice.
As you grow more effective/efficient, you'd probably be able to do a little better than this, but then your costs will also grow along with your practice. You might have more issues getting hospital admitting privs without BC/BE, too, depending on your locale, but then they do have hospitalists don't they?