If you go to the Dintrol website, you can find a list of approved garages that will do the full treatment, which is a proper underchassis steam clean, rust treatment as required, then injection of cavities and full dinitrol undersealing.
This will cost probably at least twice as much as your most expensive quote, but it is a once-done, forget it type of job, unlike the waxoyl and similar (old engine oil, etc) options, which are a messy, do it every year, job, so I know what I would go for personally
In fact, I very nearly did this to my T4 Camper Conversion last year, but decided against as I changed to my T5 instead and wouldn't have recooped the outlay in resale value.
There is also another product which is a decent DIY as well as Pro product - Tetroseal.
I know people who went for Tetroseal (plus Shultz on the arches) on their brand new RAV4s, as the underseal protection of new cars is still pretty poor, especially ones built in Japan, and the underneath of their cars still look brand new years later. It really is a worthwhile investment IMO, but whichever you go for, dealing with and treating any rust first is key, otherwise a pointless excercise.
For a standard T4, there are no splash guards or suchlike parts to remove underneath in order to apply underseal.
There will be some bagging up of components I would think (brake discs & calipers, handbrake cable, etc) to protect them which will be standard for all vehicles. You probably have an under-engine belly pan that might need removing - that takes just 5 minutes when up on a ramp, so don't be fooled by them saying they will have to take lots of covers off (the later T5 is covered by plastix shields and heatsinks underneath and is a different story!)
Being a "Proper" camper conversion from the start, it IS possible you have extra underchassis equipment such as propane tank and water tanks which could complicate matters a bit. I would imagine they would work around fittimgs like those rather then try and remove.