Ideally a job that pays well, provides lots of time off, gives you a chance to travel (car, plane, tank museums, model shows etc) and an excuse to buy lots of little tools.
If you find that job, please let me know what it is. Mine only provides the travel and that is a when and where I'm told to go and not much time to stop and check stuff out (except for the fire engines, airtankers and helicopters).
Other considerations might be jobs that provide you access to the 1-1 subject you like to model. If you like civil aviation a job at an airport might be good, if its ships a job at a large port, cars a job at a dealership or as a mechanic. Similarly jobs that provide you with tools and knowledge that carry over into modelling can be good as long as it doesn't make modelling a drag for you. I'm thinking something like computer drafting which you might be able to use to design scratchbuilt parts, metal plating or etching which you could use to chrome plate parts or make your own photo etch parts etc.
Of course it helps if you like the job too, we spend too much of our lives at work so it helps if it at least isn't something you hate, if its something you like then even better.
Personally I'm thinking some sort of royalty would be nice, nothing fancy, low enough that they are not dragging you around for the crowds too much, but important enough that you could easily swing a ride in the latest tank, maybe squeeze off a few rounds at the range. I've looked and Monster dot com doesn't have any listings. Its almost like you have to be born into the job or something.