water pump is defo the easier out of the three to do if it has overheated it might need the hg doing as well you need to have the head skimmed as well if you can, but its alot of work the block as well, they warp more than the heads having both of them skimmed will lower compression plus a thinner head gasket your asking for trouble. the water pump is not part of the cambelt of the starlets which makes it an easier job you can get the water pump from any motor factors i paid about £20 for mine and it was a nippon one same a toyota use plus the vat so for the sake of it it'll be worth doing