We have done the inside of a car and the inside of a large part of a huge trailer. The trailer was really no different than doing the outside. The car was a very complex install over a lot of custom fiberglass interior bits.
While we do a ton of ambulances I've never been requested to do anything really on the inside other than standard no smoking, oxygen in use decals, etc. There are a LOT of obstacles. Be careful removing certain things like the Ohmeda O2 wall plates, those often have the O2 lines crimped onto the back which you will not be able to reconnect if you remove the crimp. Also the face on those plates has the front valve attached with a longish brass tube going into the wall half of it with an o-ring seal. It's fine to remove that part but make sure to then protect the part still on the wall so nothing gets into the orifice and protect the brass tube on the plate you remove. There will be 2 tiny o-rings as well, if one falls or gets lost dont worry - those are just to retain the 2 screws and do not actually seal anything... A lof of the handles, vents, windows trim, etc can also often be removed pretty easily but it varies from brand to brand of ambulance.
As far as making the design meet at corners, if possible make it blend with some generic pattern, color etc where one side meets another. If there is a stripe at least try to make sure it has a consistent width where it might meet another side and tape it all up to make sure they line up before starting to apply.