Agree with others here. Definitely spend as much as you can afford on the actual tent and don?t fall for the bonus extras - which are often cheap/need replacing fast. It is a sensible move to buy one that you see set up for yourself - tug on the zips, check out how secure the poles slot together - are they wobbly and loose or do they fit nice and snug? Is the outer flimsy and looks almost see-through or does it look tough with decent robust seams with nice tough zips.
The built in groundsheet should be breathable (lots of campsites are stopping the older style non breathable plastic groundsheet floors now for environmental reasons so again worth checking). Also, there should be a decent amount of space between the inner and outer.
Look at the guy ropes and tent pegs too - some are so thin and cheap that they would need replacing really fast. There are good tents out there with naff pegs so don?t let this be enough to put you off - just consider investing in a replacement set of really strong "nail" type pegs with dayglo tops to find them in the grass. If you ever have had to pitch on rocky mountainous terrain, you will thank your lucky stars you spent the extra on decent pegs. If you have ever had to get up in the middle of a gale to repeg out a tent that has half collapsed, you will understand my rather odd obsession with good pegs..
Another factor in tent shopping is the living room to sleeping area ratio. As a couple with a new baby, we wanted decent sleeping area with reasonable head height for nappy changing etc but not needing acres of space to cook/sit around in. Now with two lively children our needs have almost reversed: head room in the living area with plenty of space and the sleeping area divided into 2 areas but not needing to be massive.. Consider your immediate needs and needs over the next couple of years to save you buying a second tent within 2 years or so.
Hope that helps