If you go for derailleur bike consider buying new rear wheel with IGH (nuvinc or Rohloff). Rohloff will give wider range which is better for off road. Pay extra for strong build will save on broken spokes. This is expensive upgrade but will save money in long run and reduce you maintenance time as single speed chains are very forgiving of bit neglect and very cheap to replace.
I'd run something like Jonny Watt tire in rear and buy extra front wheel with knobbly for MTBing. Spare wheel can be lower quality as it would only be used for commuting.
I'd run something like Jonny Watt tire in rear and buy extra front wheel with knobbly for MTBing. Spare wheel can be lower quality as it would only be used for commuting.