Hi Andy
Are you thinking of the ST TDCI, as this is the 155bhp 2.2 TDCI?
It's the ST200 3.0 V6 that Ford say is not suitable for towing.
I can not comment on the Frontera as I've never driven one, however i can the Mondeo. I've owned the Estate ST tdci model for about a year now covered 49K miles and it has been a dream. Power, comfort and economy all fantastic. We bought a Sterling Europa 600 in the spring and we have had great holidays. The mondeo pulls with no problems the torque is massive and 6th gear at 60 on the motorways is easy.
I got a Witter detachable towbar fitted and this is nice a discreet when not in use.
I hope this is useful
Mark