Guilty as charged, at-least 5 different kneeboards, and three different headsets, depending upon what I'm flying. For that matter, 3 different flight computers, several rulers, a couple of stopwatches - it's amazing how these things accumulate.
Not sure personally I'd recommend buying anything straight off, except a local 1:500,000 chart (personally I prefer the Jeppesen charts, but virtually all schools use the CAA chart so that's what you should probably go for) and a logbook.
Headsets are expensive, if the school has decent ones spend a fiver on a couple of cloth earpiece covers - they will help your general level of comfort and hygiene. If the school ones are truly awful then the reasonably inexpensive Harry Mendelsson HM40 sets are good enough for most purposes and match virtually anything else in the other seat.
Textbooks, protractor, computer, ruler, etc. Your school or instructor will have their preferences - go with those. If the school on the other hand tries to sell you a complete set, I'd buy it - since presumably their teaching is based around that set of bits.
And don't let anybody sell you an expensive flight bag. For starters a Tesco carrier bag works fine, and after that even most professionals are using something that cost £20 from Argos.
Well that's my opinion anyway.
G