Hi Paul, 
 
Thanks for your question and excellent feedback about your mattress purchase. 
 
For your daughter, at her age and body weight, we'd recommend the Origins Comfort 1000 rather than the Origins Pocket 1500. 
 
The Comfort 1000 is slightly softer in the top layers providing a bit more of a sink. The Origins Pocket 1500 is more a true medium feeling model but my concern would be it might be too firm for her. We do sometimes also find that you might need to add a mattress topper to the Comfort 1000. 
 

 
This mattress will still allow her to grow with it as she get's older. I have this mattress myself and I'm 32 and roughly 15st so ample growth opportunity.
 
You can absolutely mix and match the base. The Artisan Base adds extra support to the mattress and being a sprung base will give the mattress a slightly softer feel. You would just need to add the Base and Headboard to your basket separately because the Origins mattresses will be paired with the Origins Platform Base as standard. 
 
I hope this has answered your questions fully but if you do have any further queries please don't hesitate to contact our office on 0161 437 4419.
 
Kind Regards, Lee.