I had several, many years ago. A lot depends on what you get. There were 2 basic shapes - the early ones had round headlights and the later ones had the faired-in swivelling headlights. Of the earlier ones, there was an ID 19 which was actually quite a simple car - conventional steering and brakes, but still the whacky suspension. Not too bad to work on - other than the rust.
The later ones were more complex - especially the last of the line with electronic fuel injection and Citroen's own semi-automatic gearbox.
I'd go for something middle-of-the-road like a D Super 5. You get a relatively simple engine with a carburettor and 5 speed manual box, but you still get all the weird Citroen hydraulics.
Be aware that the early round-headlight cars used some sort of stinky fish (or vegetable - not sure!) oil as a hydraulic fluid because they used natural rubber seals. It was known as "red fluid" and it absorbed water. Consequently, their hydraulic systems gave a lot more trouble. They were, however, the most softly sprung ones so they give the floatiest ride for which the marque was famous. The later cars used green LHM mineral fluid which you can still buy easily today, and neoprene seals.
I wouldn't be too bothered about mechanical complexity (they're actually fairly simple by today's microprocessor-controlled standards), but some of the jobs take a very long time (especially a timing chain or clutch)!
Lastly, they all rust like mad. They were one of the first mass-produced pressed-steel monocoques and they were full of water traps and not very well protected in the first place. The very bottom of the boot floor and the rear trailing arm attachment points were famous for it and extremely difficult to repair properly. Sills go too - especially on the ones with "Pallas" trim - which was stainless, and attacked the mild steel to which the trim pieces were attached. Lastly door bottoms all rot out because of poor drainage.
Good luck! A fantastic car for it's day with probably more innovations than any other car launched before or since!