yeah, you'll need both. headset and monitors.
Monitors for $200-300: little powered-active monitors are the norm.
nearfield means near- aprox 3ft/1m from you, in comparison to far away HiFi that are designed to utilze room reflections,sitting 10-20ft away
in theory, these "Reference" speakers for mixing have a flat response, meaning they can keep the same volume to your ears from 20hz to 20k..which is impossible but designers try..
this never happens either.
Monitors at $2- 300, very common starting point.
1) Yorkvilles
http://www.gear1music.com/yorkvilleysm1p.html
use the Coupon.10% off.
2) You can probably get used Wharfedale Actives for around $250, like you said. Several have mentioned upgrading them as you've read. Monitors are a common gearhead addiction so ~6months to a year and its upgrade time.
Idea: You might even try a post "$150 I Want to Buy monitors" on the For sale thread and see what you get? that'd leave you $150 for headphones!
and i just happen to have a brand new AKG271 for sale...at $150! maybe??
I went Passive.

$150 used Mint Tascam PA150/300w amp /Monitors $150 Mint BW DM303 & some very non-mint $125 JBL Control 5's that play fine. But its been about 6 months so I maybe getting some DYN BM5A's,BM6A's here soon... i'll sell you my stuff for, like $700?
