Consider this; here in the USA we have spec Miata, essentially stock engines and people routinely spend 5-7K on professional builds / rebuilds. Also note if you have a Formula Atlantic (4AGE) a simply rebuils will cost 11-13K.
Now do I think this is insane, sure but to each his own. To add a little more perspective if you wanted an all out vintage 1200, you'd have 8K easy in an engine, 2K for a tranny, 2-3K on suspension etc...........it would be real easy to get over 20K in no time...........and the final bit of fun, how about 8K on a single cylinder engine for your Shifter Kart.

Tom