fill the cooling system and then run the engine for 5 mins to see if any water leaks out (leaks are much easier to find once pressure is created in cooling system). you'll see the drips on the floor (if the floor is dry when you do this).
if the head gasket has gone, the best thing to do is rip the head off, get it skimmed, and then refit the head using a new head gakset kit. if you can do the labour yourself it wont cost much to get the head skimmed somewhere (£20-40) and buy a head gasket kit (£30-50).
if you cant do it yourself, then ring aroung all the local independant garages near you, i'm sure you'll find someone who will do it for £200, plus you then have warranty on the work, and a nice receipt for your document wallet.
changing an engine purely due to head gasket failure is not neccessary or wise. it will end up costing you more, and you never know what you're buying, and you have the milage discrepency issue whenever an engine is changed, so it devalues the car, costing you even more in the long run.