When going through your taining its best to stick to issue kit. So you don't pee off the DS, most of the kit issued now for tours is what you will need to do the job. But guys do buy stuff to make life more comfortable.
Beware. If you don't know what you're buying, you may well attract some derision when you arrive in your Flecktarn sniper's smock with 284 pockets and a built-in GPS. Also, whatever you bring you will probably be made to use the issue kit at first to stop you gaining an unfair advantage (better-off recruit buys three pairs of boots and four sets of uniform; after an ex he just gets another set out whilst poor recruit who can't afford to kit himself out has to stay in the ablutions until 0230 scrubbbing his kit clean).
Definitely keep to issued kit through your basic training, this gives you an opportunity to learn and get to grips with it.
We had a lad who brought his own webbing (airborne) to only found out he couldn't use it. Also had people turn up with non-issued boots they couldn't continue to wear them so my advice is to keep to issued kit untill you've passed basic.
Obviously you can make things a bit easier for you.. Such as buying base layers, waterproof bergen liners... but no need to go and spend Â£Â£Â£'s