Tony
I agree that you should take a look at David Hardcastle's book. You should also check out
www.v-8.org.uk where you will find lots of advice about performance enhancing bits & bobs.
I got my carb, manifold and air filter from e-bay but it may take a while for the right bits to come along at the right price. Remember all the "hidden" extras like new thermostat housing, throttle linkage etc. If you put in a drastic cam then you may have to think about modified springs, cam followers and possibly roller rockers. All are reasonable prices but they will affect your final bill!
Best advice is check the prices from the sellers already listed - you will be surprised by the differences. Also try
www.pegasusparts.co.uk .