Daniel welcome to the forum and congrats on your find in the basement-- The Fenwick is fiberglass as opposed to the graphite fly rods more commonly found these days.
But in it's day (1970's) those Fenwicks were highly prized and there's a bunch of folks that still enjoy fishing them. My first fly rod was a Fenwick and it still sees some action, and as Rip Tide said it's a great small stream rod.
You've gotten excellent advice already, but i'll add to the confusion
1. I'd get a 7 1/2 foot knotless monofilament leader tapered to 3x and a couple spools of monofilament tippet material in 3X, 4X, and 5X. Add about 2 -2 1/2' of monofilament tippet material to the end of your leader depending on the size fly you're using (see the FAQ section for a discussion of leaders, tippets and fly size). There are other great options as well including using a furled leader, but tapered mono leaders and mono tippet are widely available and relatively inexpensive.
2. You'll want a selection of flies that will match different food items in your local streams and to cover different layers of the water column. Your local fly shop would be a great source of info for patterns, but a good example would be something like the Orvis assortment of 20 flies for $9 that appears as a banner ad on this site. With a couple of dry flies, a woolly bugger or two and some bead head nymphs you'll be off to a good start. Also in the FAQ section there's a list of gear you might want to consider for trout fishing and it includes a list of some popular and widely available flies that cover a lot of different situations on trout streams.
3. Yes, it's possible, both using small lures with a fly line and fly reel and 'fly casting", and using the fly rod as a spinning rod matching it up with a light spinning reel and 4-6lb mono. But fly casting lures can get dangerous if they're equipped with treble hooks, and it likely won't be too elegant. And also realize that there are many choices available for flies including bead head or conehead streamers that might work even better for you than a lure.
4. I wouldn't use your rod for giant tuna, but your rod should be able to handle any trout you run into. To land a large fish that might weigh more than the pound test of the tippet, you'll most likely have to let the fish run and take line, and you can apply a little resistance by using the fly reel's drag or by controlling the resistance by pinching the fly line with your fingers as opposed to having to wrestle the fish with your rod on a fixed length of line. With a truly large fish there are a zillion things that can go wrong, and some times things just don't come together for no apparent reason. But the rod is going to be the least of your worries if you're hooked up to a monster. Stuff like keeping the line tight to the fish, maneuvering around obstructions, well tied knots, and not exerting too much pressure on the light tippet will be the bigger issues. And at any rate, you'll also come to realize that the big fish that get away are the most memorable.
Good luck on your adventure, you 'll have a blast-- and that rod is a classic!