First priority: helmet, (cheap is OK as they do not get much high speed impact damage)
followed by gloves and boots.
Wulfsport do cheap one or ring up importers and see if any last years models are going cheap.
Buy trials not motorcross boots as the latter are too stiff.
I work on the philosophy that the only too points of contact with the bike are hands and feet, and hopefully you value your head.
Trousers are next on the list of essentials again last years models are just as good. Finally sweat wicking football shirts are just as good and can be had for less than a tenner.
Don't break the bank just ride!!