Not entirely what you are after but I've had good experience with the DaryTek Vigor 3300V firewall device which can take upto four ADSL (separate) modems and load-balance between them.
http://draytek.co.uk/products/vigor3300v.html
For modems the Vigor 100 ADSL/ADSL+ modem is very good but strangely enough I can't seem to find it properly on the website but it is listed on the products guide page. If you use these then the configuration for them is passed through from the 3300v.
It's been about nine months since I last purchased and set these things up but the costs then (as far as I remember) were about £600 for the 3300V and £100 each for the Vigor 100s. They are all readily available from many online IT suppliers.
Well worth a look at.