Ok, first, my condolences about your parents.
I was in the same boat as you about 15 years ago. A few things got away from me too and got damaged.
Now the good news. Rain water is pretty much purified water. This is very good news because if any salt water gets into any electrical; it's done. Water does not get into wires so the only thing that corrodes is the connection. Even that rarely happens because most connections are plated. So good news so far right?
Now the not so good news. I didn't say bad new because it's not really that bad of news. The stepper motors that run the gauges are kinda fragile. In fact they go out all the time with truck in climate controlled garages.
More good news. There are people out there that can replace the whole gauge cluster with a serviced one pretty cheaply.
About the interior and the not so good news. You're more than likely going to have to pull the seats out and have them redone. The carpet and padding and headliner is also going to have to be replaced. That will not be cheap but not too bad either when considering new truck costs or even used truck costs.
You are also going to need to have the cab ozoned. A lot of high end detail shops have ozone machines where you can have this done. This will kill all of the mold in the cab and make it smell good again.
Even if the puter is bad it not crazy expensive to replace. Not cheap but not 5 grand either.
All in all without seeing the truck, this things is VERY much worth saving and will be much cheaper than you think.
Good luck and let us know how things go.