The most important thing to consider when buying a bed is the mattress. You will need something which will give good support and last a few years - I would highly recommend a Dunlopillo mattress from experience - it keeps its shape, comes with a 10 year guarantee, great for allergy sufferers.
Once you've bought your mattress, make sure that, if you put it on a slatted base, the slats are no more than 6cm wide or more than 4cm apart. This ensures sufficient ventilation, but prevents the mattress sagging through the slats.