Hello and welcome! Very nice colour choice! Good idea on the protection, cavity insulation can be done at home if you have access to a compressor. Waxoyl (wax oil) type products will work through a schultz gun and can be sprayed into chasis rails, nooks and crannies. There are companies that can do this as well if you don't have the gear, usually off road / land rover type places are a good bet as they try and waterproof as well as possible for water submersion.
If you need something for the exposed areas i.e. floor pans etc. then a hardening substance like underbody seal applied with a paint brush work well. There are a variety of subtances out there, jotun has a good reputation and are UK based. Anything that's for marine use is a really good option!
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