Two things you will use after you get your PPL are a GPS and a headset. Get a good one of each. Oh..... another thing is one of those rulers with nm and a protractor for course on it.... I think that makes three things!!!...Oh! and a map, which makes it 4 things.
If I had to choose 1 item it would be a good headset. It will protect your hearing, it will ensure that you hear the instructor clearly and will last many years after your PPL if you treat it well.
Most of the other stuff you can do without. Special bags, special watches, computers etc. are not really needed, you can buy a regular bag and watch etc. You can often buy the books secondhand and maybe even borrow them (try a wanted ad on the club noticeboard).
I do think that there is a tendancy to think you need more stuff than you really do....and of course the club won't mind selling you things you won't need.