How do I decide which mattress would best suit us?

We have never bought a brand new mattress, as we've not had the money, and have overlooked the benefits of having a good quality mattress to spend half of our lives on!
Our current mattress is quite firm, but the wife (5'8' side sleeper' and 9st) finds it uncomfortable. I quite like it, but as a side sleeper to avoid snoring, I can wake up with a sore shoulder (6'2'' 16st).
We are both 27 and expecting our first baby in January, so would like to ensure that what little sleep we get isn't hampered by feeling uncomfortable and hot!
Our budget isn't huge, less than 700 as we will also need a new king size frame, but see this as a good investment.
Any advice greatly appreciated, thanks!