What type of sander should I buy, a round orbital one (which seems to be the new fad), or a rectangular one?

Also what is the difference between one costing £20 and one costing £150, other than the motor will last longer?

get a makita palm sander, and a makita 4" belt sander. the two will prob cost about £200. orbital sanders are rubbish. with the two i suggest you have fine work and large areas covered.