Our trailer (carriage trailer, 3 carriages, stuff for ponies camping for 5 nights, 6 pop up stables) is a full up 3.5 tonne loaded- our range rover and 7.5 tonne lorry don't find it easy uphill. HGV and defender pull it fine.
I suspect I have the same trailer as yours - Pegasus extra long base with day living..? I tow it with an 80 series Landcruiser Amazon which hardly notices it's there.
I would go for a 110 defender you get the power but also the bit of extra length to balance things out
I have a defender 90 who is affectionately known as "the tank" for a reason! I pull my rice trailer & 1 16hh idxtb & it barely feels like its pulling anything !!