I have got the Exmoor middle seat with headrest in the front for one of our kids (my mother can also fit in it), and one side facing rear seat from Exmoor for the smallest, stops fighting in the back too. We go on holiday towing a caravan, 4 bikes (1 on the roof and 3 in the back), 2 weeks of camping gear etc. You could go with 1 forward-facing rear seat and 1 middle front seat. You would need a 3 point harness belt in the front seat though.
Mine is a 1991 Defender 90 by the way.