Yeah, I'd use OpenCart, and then if you want a blog to drive traffic, install that to for example yoursite.co.uk/blog
It's better for SEO to install it to a directory off your main domain than as a sub domain, as that'll give more "link love" to your main domain.
Forums are normally installed on subdomains as they link out a lot, which could otherwise have a negative impact on your main domain's rankings.
Subdomains are treated in part separately.
Any decent web host will provide a one click installer so you can have OpenCart up and running in seconds.
If you're curious about the other options available, the Softaculous site (arguably the best one click installer out there) has ratings and reviews for the most popular options:
http://www.softaculous.com/softaculous/apps/ecommerce
OpenCart and PrestaShop come out tops, although I'd recommend the former.
You can even demo them there to see which you like the look of, here's the OpenCart one:
http://www.softaculous.com/softaculous/demos/OpenCart
I hope that helps
Good luck!