£2k? maybe £800-£900 with labour from a garage. Very good mod, gives good Gaines but poor bhp per £ though.
Headers aren't value for performance. They just aren't.
They're a total PITA to fit with the engine still in, it's possible but expensive.
Headers are something you should consider alongside cams as part of a proper N/A build, they're unlikely to give you more than 10bhp for your £2k otherwise. The fact that you've asked this question suggests that you shouldn't consider such a build until you do a LOT more research.
DB