The kit will mostly consist of the blocks, bolts and hardware to realign bumpers etc. We have to check all the little details when the truck is in the shop. As for engineering, we have two ways to do it. One is to have one vehicle checked all over with stress test, driving test etc by the TUV (DOT) and get a "master"report on it you can give the customer who installs your kit to have it engineered for about 50$ after installation. This involves around 10-15k$ upfront for the masterreport and you need to have ISO9001 standard in your company etc.
The other, easier for us, way is to do single vehicle approvals with an TUV engineer where they basically check the fitting of the kit, do a short test drive and we provide them with a material declaration of the parts. This single approval is about 150-200$ per vehicle and is often used over here. This is speaking for Germany, though. Other european countries differ, France for example doesn't even care about any alterations to a car, Austria does allow only very little and you even have discussions with a masterreport
As for tires, we have a complete mess since they started to take emissions extra serious (Germany wants to be leader in this
). We're usually allowed to go a max of 8% bigger in circumference from the smallest stock tire on everything that is Euro3 or better. So the Hilux being Euro4 we only can go to a max of 275/70R16 or 31x10.5 without changing gears to correct the total gearing. Everything that is less or Euro2 is pretty much easy. I have my GQ Patrol engineered with 35's, no problem being completely Euro0. Drawback is that the road tax is measured by Euro0-5, the lesser the number the higher the tax. So for a Euro0 TD42 GQ you look at 3000$ road tax a year while a Euro4 would be 800$/yr.
And the airbags, well, we have engineers who know and understand how airbags work and they do not see a problem with them as long as you do not alter the frame in front of the sensors.