'Speciality' or more commercial oriented vehicles including the 4trak and the trooper (which is also a chassis construction BTW, or at least the older ones were) have a plated weight on the VIN plate which gives the maximum towing capacity. 'Normal' cars don't and the towing capacity is based on the curbside weight - as previously stated this is 50% for an unbraked trailer and 85% for a braked. (I didn't know that either until Mr Harvey mentioned it!
I have no idea how the plated towing capacity is worked out, but I suspect its a combination of stability, chassis / towbar construction, vehicle weight and COG, and will be a factor calculated at the design and test stage. Most pickups for example have a pitifully low towing weight - I believe the new Navara can only pull about 2.2T for instance - and I would suspect that this is to do with the lack of weight over the rear axle and generally more flimsy construction as a result of not having a proper bodyshell at the back.
I would also echo the comments about settling for a car thats 'just good enough'. If nothing else if you're straining the car all the time you'll just end up killing it. I also wouldn't really want to put a nice car anywhere near the sea! Buy a Rover 418SLD and a braked trailer and be done with it!
(85% weight is about 960kg if memory serves and it used to tow our Microplus
without too much trouble.)