You're going to get people saying you should buy this gun definitely, because they own it. While it's always best to get info from people who actually use a marker, they can also come with a bit of bias so keep that in mind.
Most things ego / etek / geo(1) / DM <2009 can be bought for £250 give or take.
I'd look at Ego 07/08 or DM8/9 's for your price budget. Great high end markers for relative pennies because they're 'old'.
Whichever make or model you go for is personal preference. There's a great
newbies guide to buying a marker sticky in the newbie area of the forums to have a look at, if you're not aware of the general differences between each type and make of gun.
Equally, a newer marker like the axe would be a great buy as they've a really strong following. Or one of the more mid range etek / etha's within the planet eclipse range.
It depends whether you mind an older marker or not.
Personally I've just got back to paintball after a long stint out and picked up a DM9 (for pennies). The way I see it is i'd rather the high end reliability and build quality etc etc of a top marker but a few yrs old rather than a 'lesser' gun because it's a bit newer.
That being said, the new eteks, ethas and axes are supposed to be sublime! So It's personal preference really. As with most things in paintball
RE: DM8 and DM9 pricings...Being a second hand market, prices are generally dictated by the subjective allocation of prices of what the seller wants for it.
You can't say DM8's are worth this... DM9's are worth this...
They have a general ballpark area from x-y and sometimes they can overlap. For example a sublime condition and upgraded DM8 could be going for a price of £250, but a standard DM9 could be going for £200.
It all depends on condition and ultimately what the seller thinks it's worth and wants for it. Whether the seller wants a quick sale or is happy to wait is another factor.
As a general rule of thumb - I'd say DM8's were going for 150 - 250 and DM9's 200 - 300.