Everything can rust! Being a DHC, you've got to have a good look at the floorpans/inner sills. Otherwise they liked to rot the front turrets and inner wings under the seam with the outer wing/fender, the B post/sill (rocker) joint, and any other random points... '79 should be the 5 spd (and it has the " 5 spd bonnet") which is a sturdy if slightly obstructive 'box.
Stock suspension even new is oh-so-very-soft. The common after-market sell most things, there's Rimmer Bros in the UK, but they stock some junk too, plus Moss/Triumphtune. I gave up on getting springs off the shelf, they were just plain bad quality and went coil bound before getting to the bump stops, they sagged, and were just generally bad - in the end I got custom made from a motorsport supplier in the UK. This did do some rough events though.
One thing to note if / when jumping to V8, they had a different front subframe, mounts were different and they had some internal reinforcement. That said, mine had the stock 2.0 subframe with mounts welded on for the V8, and never gave me any trouble.
Cooling - even with a big engine bay and vents in the bonnet, traffic in the summer was always stressful (whether with 4 or 8 cylinders). I ended up with 2 huge electric fans and an uprated alternator to run them, which just about kept things cool. The water pumps on the 4 pots are fickle and fragile - there was a reliable electric pump solution coming out around when I stopped running mine. The 3.5 V8 is stout enough in these, I had 2 failures with mine, one was a stuck-open oil pressure relief spring (complete random), and the other was a stripped dizzy drive gear - at 8pm on xmas eve in heavy traffic on a long trip to see family; I spent xmas day eating a cheese sandwhich and stripping all the timing gear from the engine, called up and old mate and got drunk, then as soon as the insurance companies opened on the 27th transferred the insurance to my 309 Gti and headed off again into a snow storm through Wales..
I'll be honest, some of my info may be horribly out of date - mine finally lost the rust battle back in 2009- ish, having had a good run from 2001 with me, serving as an all-season daily driver and navigational rally car.
That inner wing rust? About 2 miles after this photo...
..this happened (that inner wing seam should be straight, it's gone inboard about 1" after a big bump...)