Every area will be a little different.
Some states, cities, and/or counties will require a business license. I have an AZROC contractor's license as well but only because I have it for the welding shop which must have it for anything outside of small shop fabrication jobs that don't involve field work or contracts.
Then, insurance. You better find a good agent and check that one yourself. My Gen Liab/Prof Serv Errors and Omissions with worker's comp, on site auto, and who knows what else costs me about $1000 per month. Don't try to cut this one short, it will come back around to bite you. Oh, and that is for $2 mil with another 2 umbrella. You may be able to get by with less but it will limit the jobs you can get.
Then, any advertising? Website, business cards, shirts and hats with business name, etc? You will need to register for a trade name and/or get incorporated, LLC is very common but mine is an S-Corp, again because of the years with the welding shop.
Land line phone with answering machine and fax, yes, there are many companies that still want to send info by fax.
Working for yourself; inspection tools of all sorts, all the applicable code and reference books you will need, lap top for reports, hot spot for locations where you don't have access to someone else's WIFI.
Safety gear: hard hat, steel toes boots, vest, fall protection, etc.
Start adding it up. It is not cheap. Not if you want to do it right. I'm sure you can sneek into some jobs without having all of this, but one day the liability will pop up and you will find yourself paying one way or another.
I have probably forgotten much but I'm sure others will chime in.
He Is In Control, Have a Great Day, Brent
Just my two cents...Is it possible for you to work under the umbrella of a weld inspection lab/service? Less expensive for you until you raise the capital to go it alone and it would reduce your liability exposure as you would be under their policy.
This is my game plan for when I retire in a tad over 5 years, plus I get to sit in on all their training classes for free.