The clutch cable could also have stretched. If you look at the pedal and it's not cracked, have someone watch the arm on the transmission that the clutch cable attaches to while you push the clutch in. If you put your foot to the floor and that arm doesn't move at all, then you know it's a cable or linkage issue.
As for the stalling on the highway, it could be lots of things, overheating is certainly one of them. Most people selling a car will fail to mention "Ya the temperature gauge went off the end but I kept driving anyway". Have a look at the dipstick and see if there is oil burnt to it. Usually a good sign that it got WAY too hot. But if the car starts and idles, and doesn't get really hot, but only dies when it's moving at speed, my first thought would be a really plugged up catalytic converter. If it still has the original cat in it (it'll look nasty as all get out lol), it may be letting enough exhaust gas through that it's fine at idle, but on the highway at higher RPM for extended periods, it can build up so much back pressure that it chokes out the engine. Also see when the timing belt was last changed. If its way overdue it could have stretched and skipped a tooth, which can cause all sorts of havoc. My advice is free, take it for what it's worth lol, but I have had a few of these cars over the years, and mine are not usually that nice so I run into all the weird issues ha ha