I have a towbar installed on mine. I use it mainly to mount a very solid bikerack to fend off would be rear end collisions, and whenever I can, take a light wooden trailer out for some camping fun. It also helps when moving house, or transporting firewood.
This is the second ST185 I have had a towbar on, so I say the following from experience:
there is absolutely NO WAY I would ever seek to tow a car on a trailer, behind me. in a nutshell, here's why:
for starters, managing to find someone game enough (and skilled enough!) to actually fabricate a custom jig, AND put it on, was 'challenging'. mainly because: with the fuel tank where it is, and the monococh shell/chassis, there is in fact nowhere to install it. I'm pretty sure it fails the ADR's here in NZ (the legal rules for towbar installation). but no grief, so far, so good.
the jig/hitch/towbar is NOT rated for that kind of weight.
and finally, but actually the biggest point of consideration for me, is it really DOES put a strain on the engine. fuel economy speaks loudly to support this. admittedly, I live in a place where you can't find a level road, ie very hilly, lots of inclines, and corners etc, BUT ... as we all know, these money pits are maintenance intensive, and ... I simply just can't afford to blow bits unecesarily.
hope that helps. (and Im super impressed to find so many others with a hitch on their ST185. way cool.)