"Does this look a good spec to replace my desktop computer or am I missing some important components"
To answer your question we would need to know the specification of your desktop PC. It also depends on what you intend to do with the lappy. Despite claims made by manufactures, few laptops will match the performance of a fairly well specced desktop when it comes to gaming, and DVD authoring\editing.
Your laptop looks OK for a general purpose machine, and for the extra four quid I would opt for the larger drive.