what tools are considered necessary in order to do proper work on a locost, or any other car or motorcycle which might find its way into my garage?

CaLviNx posted a good starter list in this thread:
http://www.locostbuilders.co.uk/viewthread.php?tid=44950&page=2

Depending on what you are working on you might get away with just Metric sockets/spanners. I'd also add an engine crane and axle stands to the list.

Once you have that little lot you can start deciding on the more specialist tools as you go along.