Old Kalkoff cross bike, worth restoring or not?
That isn't a cross bike. First, make sure the bike fits you. If it doesn't, its not really worth restoring (give it to someone who could use it). It looks to be spec'd with relatively cheap stuff, so I wouldn't sink a lot of money into it. 

The first thing to check is if the frame has any major cracks or damage. If that is in good condition, check the headset to make sure its working smoothly, that the fork is functional (it probably doesn't work well, but if it bottoms out automatically its relatively useless), the pedals turn smoothly, tires hold air, wheels are true, brakes work (probably will need new pads), shifters work, chain is OK, derailleur not bent. A lot of this will likely require elbow grease and real grease/oil rather than new parts. 

If the fork is in bad shape, you can likely pick up a rigid fork for not much money, but you need some tools to install it which probably means a trip to the bike shop or local co-op. If you take it to a bike shop and ask the guy to give it a safety check, he should be able to quickly determine what needs to be replaced for a few bucks. As for aesthetics, well, not much you can do there. 

In all likelihood though, all you're going to need to do is oil the chain, adjust the brakes , adjust the derailleurs, throw on some new brake pads, adjust the cables (some may be broken and need replacement), pump up the tires, adjust the seat and handle bars and you're going to be riding off.