Hi mate, welcome to the club
First thing you you need to do is check the chassis at the weak point between the cab and tub, there was a recall on these and you need to check you can get this sorted first.
is the chassis got holes in it or just surface rust? if surface rust then you just need to cheam it all up and spay on dinitrol
Then when that has set and hardened, get buzzweld Lanolin and spray the underside of the car a couple of times, You can spray lanolin every year, its really easy to do yourself.
see the chassis snap here: https://practicalmotoring.com.au/car-news/nissan-d40-navara-caught-chassis-snap-drama/