With £500 to play with youve got things like ego 11s/DM12/DM13/Axe/Geo 2/2.1/possibly 3. Id personally recommend an Axe or an Ego 11. Both Empire and Planet have brilliant customer service and all their markers are simple and robust.
Probably steer away from dye, love the look of my marker but has been unreliable, I spent a fortune on getting it working and worked fine for a while, now bolt is sticking forward!
Sadly mate NT11's are notorious for unreliability. The DM series are far better, I know people who swear by their DMs but overall NTs are universally avoided due to their relability issues