Actually, it would make more sense to equalise the tariffs of SUVs and cars. Currently, the import tariff of SUVs is only 5 percent, while for cars it is 10 percent (or 15, can't quite remember). Because of this reason, imported SUVs (read: all except the Ford Territory) are often cheaper than their road-only counterparts, making them more attractive to prospective buyers.
Furthermore, a tax on SUVs could potentially have socioeconomic implications. Could it be that such a tax would be disadvantageous to low-income earners while more affluent types can easily afford it? Clearly, for the purposes of fairness the tax should be proportional to the cost of the SUV a la the LCT. There are also other problems. You suggest a tax break for farmers, but do you realise that may so-called city-slickers own farms, in some cases for no other reason than as a 'weekend retreat'. This fact represents a loophole in the legislation for people who will register their SUV with their farm address and then drive it around the city for 99% of the time. Therefore, it seems that the tax system you suggest would be a dumb idea.