Responsive themes for my bedding store on 1.5.5.1

Hi,
I am using opencart 1.5.5.1 default theme for selling handmade customized beddings.

Now i want to move to responsive theme and want to have top mega menu and good looking site as well with sale / sold etc on product images.
but do not have any idea about -

1. what all features should i check while choosing theme?
2. should i go for Free? or paid themes?
3. which premium theme is good for 1.5.5.1

how much time will it take to upgrade. I kept making small changes in code as and when needed and never noted down. It seems i need install it afresh as coding changes might disturb site with new theme.

Please share your opinion.

Thanks
Pooja
http://www.LinensnCurtains.com