try to get someone who knows what they are talking about so you get all the right fittings, be aware that you'll need to buy roof bars as well (unless your car has roof rails!) and get the biggest you can- if you don't you'll wish next year that you had!
Will you have somewhere to store it 'out of season'? ours is 'hung' by 2 big loops of rope in the garage roof.
Oh and be aware how much taller you car is with one on - dh has nearly sliced the top off the car on several occasions, trying to go in car parks that have height barriers
I just bought some cheap Paddy Hopkirk bars for my Zafira (no roofrails) from Halfords yesterday, lucycat - Thule ones were close on £100, the PHs about £40. Haven't tried fitting them yet, hope I don't regret going DIY; I never priced them at Vauxhall.
(The roofbox goes on DH's car which does have rails)
Worth checking out users' reviews before buying as some are much easier to use than others: http://www.autoexpress.co.uk/producttest/64382/best_roofboxes.html
Also have a look at www.roofbox.co.uk - we've just fitted ours which came from there (a Kamei Delphin).
We decided not to buy from Halfords in the end as found better value for money even with their discounts.
We've got a roofbox that opens from the back (so the hinge is at the front of the car iyswim). Although it does have arms that 'lock' it in the open position, I find that being short I am unable to reach high enough to do it. If I had known this I would have bought one of the ones that open from the side because then you can at least open the car door and stand on the sill to give yourself a bit of extra height.
Apart from that it's great and it was surprising how much camping stuff we managed to get in it last year (dh was there to pack it!). Just a bit annoying that it was intended for more everyday use (as ds's sn buggy didn't fit in the boot of our car) and that didn't happen as I couldn't manage it on my own.