i don't know where you got your statistics
they certainly are not those quoted on the kia site, or the excellent kerbweight site.
2003-2007 sorrentos have a max tow weight of 2800kg, perfectly adequate for towing a 505 and two horses
http://www.cuddles.abelgratis.net/kia.htm#sorento03
the 2007+ sorrentos have a max tow weight of 3500kg, identical towing capacity to the daddy of the tow vehicles, the disco.
http://www.kia.co.uk/New-Cars/Range/SUVs/Sorento/Specification/Technical-Specification.aspx
turns out the auto models have the higher capacity, the manual models have a max tow weight (you are looking for braked trailers) of 3000kg, still legal to tow even a 510/511 and 2 horses).
freelanders, depending on engine, have a max tow weight of 2000kg, illegal for towing a 505 but legal for the super lightweight trailers like the chevals.
This website is a super reference guide to tow weights and is very accurate
http://www.cuddles.abelgratis.net/kerbweights.htm
and remember, the MAX tow weight of your vehicle must equal or exceed the max LAIDEN weight of your trailer, whether you are towing it full or not. Former NR member BlueWicked did have a sticky on this subject which i think is still in place...