Absolutaly critical before you can get under any hydraulic Citroen. I have a pair of 6tonne jack stands, and that's a garage sized (ie BIG) trolley jack. I've had a couple of those small 2+tonne chinese trolley jacks, they all collapsed/bent/died under the weight of a hydraulic Citroen.
pic of screwdrivers
Screwdrivers, these are one type of tools that I find a complete waste of time if you buy cheap chinese tools. The ONLY way to go is decent quality screwdriver sets. Stanley screwdrivers I find OK, but even they have dropped in quality in recent times :-(
A variety of pliers will be handy, start with some cheap (but not real garbage) chinese pliers. Get a variety, side cutters, pointy nose, pipe wrenches etc... If you find you break the pliers, replace that pair with some good quality pliers (I find I lose/butcher more tools than I break). Vice grips are the best sort of locking pliers to get. Cheap ones are crap (though ok if your going to welding them to sheared bolts etc...)
A good start is a set of 3/8ths sockets, they aren't huge and aren't small. Over time you'll find you'll get a range of 1/4" and 3/4" sockets & ratchets. Also numerous different length extension bars, and tube sockets are handy. You'll soon find which sockets are the most useful. I find I use the tiny 3/8th's set for almost everything (as they 'fit' everywhere).
We have a local market here that sells chinese made tools quite cheaply, they aren't garbage (not brilliant either) but very usuable. A good start is two sets of spanners, these sets range from 6mm, and step through every size upto 32mm. Why two sets?? Well while working on Citroens you'll find you need 'special tools', so a variety of these spanners you'll cut, bent, twist, and basicly butcher for different jobs. Try to keep one set of them intact. It's cheaper to buy an entire new set of these cheap spanners than just one spanner, so over time you'll end up with a huge stack of 'em. Some offest ring spanners you'll also find handy.
OK, so what tools do you need for restoring cars??? Well, your toolkit will gradually grow over time to be quite large. I started working on cars when I was about 13 years old (bikes before that, pedal & motor). I slowly gathered a few basic tools back then (mostly from my father buying them for me in desperation so I wouldn't lose his). Well that theory never worked, I still lose & break his tools to this day :-). Actualy he's kinda happy with the way things have worked out, as he now borrows more tools from me than I can from him ...