Insurance would be HUGE...both health and liability. Then you would have technology costs to pay for software licenses and the computer capable of running the newest software. You also have to be disciplined about taxing your income since you don't have an employer to do it for you automatically any more. It's just a big, fat pain in the ass.
Bah! Professional liability insurance may be more affordable than you might think - could only run 1.5% +- of your gross (obviously depending on your actual prof services). Health insurance would be a b!tch unless there is a working spouse carrying bennies. Software licenses would be a drop in the bucket - and fwiw, many in civil arena usually already running acad or whatever at home on a computer fit to do so. Asides from that, invest in an accountant. write off the floor space being used as office space in your house, your car / truck as business vehicle, etc.
I think its the best idea going, and I've considered doing this myself, but capital cost of needed surveying equip would be the prohibitive wrench in my plans. A civil I know who does this very thing has talked to me about possibly forming a consortium of home office engineers / surveyors. you don't even have to be in the same town per se. anyways, looks really good on a resume too (that you ran your own company), for when the economy does come back around.