Welcome to the brotherhood. Sorry to hear your dad passed before you could share a 914 project with him. But I am sure he is smiling knowing you got one to restore in his memory.
Looks like a pretty decent car to begin with. Barefoot is right on the money regarding any/all rust issues right from the start. I would empty the car out as much as you are able, trunks, seats, carpets, glovebox, all that stuff is easily doable. Then clean the trunks out as best you can and vacuum them. Then use simple green or something similar to wash them as best you can so you can actually see the condition of everything. Vacuum the interior out thoroughly.
Put the car up on some ramps or jack stands, (not the Harbor Freight death stands) and of course put some blocks under the car too before you crawl under there. Pull the side rocker panel covers off so you can see the actual longitudinals where the jack posts are attached. Really inspect those closely. Once under, and with a good light source so you can see everything clearly, begin probing and poking at the floors to look for rust, holes, fiberglass patches, etc. Check all of the suspension mounting points front and rear. Check the area around the steering rack and where the pedal cluster attaches to the floor. Check the base of the engine firewall where it meets the interior floor. If you can, get some degreaser or some kind of spray cleaner and spray the underside and then wash it off. Here, the higher you can get the car in the air the easier it is to get it clean. One of those home pressure washers would be great, (wear safety glasses through all of this work).
Of course, I would also spray some simple green in the engine compartment and let it sit. Respray and maybe get a soft bristled brush to agitate the dirt and grease and then you can rinse that off with a pressure washer or even a garden hose.
Look under the battery box and in the hell hole for serious rust. Also check the other side of the engine compartment for rust while you are there.
I think that will give you a good idea what the condition of the tub is and you can post a ton of pictures and people very familiar with these cars will be able to identify potential issues for you.
Cheers,
Michael