I've owned many Volvos with the K-jet system. That and the dashboard are the main differences from the later cars. I had to replace the fuel pump and O2 sensor eventually on a couple of them (no difference there relative to the later electronic injection cars) but have never had to replace any of the big ticket items (warm-up regulator, fuel distributor). It really is a simple system and if it works as it should from the getgo you should be alright. I drove some of my cars for tens of thousands of miles before flipping (as in selling) them but the current daily driver is a 1980 245 w/M46 purchased with 123k in 2000. Now it has over 280k and aside from the main fuel pump and filters (fuel filter in the engine bay far easier to change on a K-jet than the under the car set-up) I haven't touched a thing.
The motor should start up cold with just a few crankshaft revolutions (cold start valve working), idle cold slightly elevated (auxiliary air valve working), no stumble cold and proper running when warm (warm-up regulator working and proper fuel pressure). There are the issues same as later cars--manifold gaskets, engine ventilation system clogs, etc.
You do not say what transmission the '79 has. If it's an auto it will not have the overdrive unless it has been upgraded already (straight forward swap). If it's an M46 it should shift into OD when switched on - sometimes with a very slight delay - a second or two. I personally prefer the M46 over the 5-speed M47. Whether auto or stick it will have a rear end gear ratio a little tighter than the later cars.
The rest of the car is essentially the same as the newer ones. -- Dave