I know it gets asked a lot but..... I would like your advice on what you think the best CMS for this site- The home page looks a lot different to the internal pages- Home page http://www.fco.gov.uk/en/ Internal page example 1 http://www.fco.gov.uk/en/travel-and-living-abroad/passports1/ Internal page example 2 http://www.fco.gov.uk/en/news/ The internal pages vary throughout the site.
I've never seen that Alterian CMS, is it paid? Hate to say it, but this company called iJoomla actually has some pretty quality magazine plugins.
custom will probably be lighter anyways. But maybe less ergonomic for administration and stuff like that. I've experienced that.
Go for the custom, thats always the best solution, if you can afford that. Hire a professional team of developers and start building it what exactly you need.