Had it been a MGTF or a Freelander I would have said avoid like the plague. But many of the head-gasket problems (I believe) are due to the engine having to work too hard. In the case of the Freelander many people bought one for towing & the engine just wasn't robust enough; in the case of the MGTF the owners used them for hard accelleration (not surprising given that's one of the attractions of a roadster).

Many 75s were bought a a cheaper alternative to a BMW / Jag & were put to harsh long motorway use but equally many have been driven sedately by people who appreciated a well built (by BL standards), comfortable vehicle, representing good value for money.

For £700 it sounds like a good buy PROVIDING there's no tow bar or evidence of one

The Rover 75 is a wonderful car beautifully refined and a nice handler (though not over stiff like most modern cars) as for HG failures, get the water system pressure tested by the AA before buying. If it passes that then there's no reason why the gasket should fail now - it's lasted this long so should be a gooden, keep an eye on the coolant (at least weekly basis) and change it every two years to keep corrosion to a minimum, it should prove to be reliable, K-series engines hold very little coolant are are very light this is why they cannot take abuse, on the plus side you get almost instant heat in the winter and fuel economy is good as the warm up cycle is very quick.

If the engine has had a HG repaired and the mechanc used the later multilayer gasket and (even better) the all-steel "crank ladder" upgrade then the engine will be very robust.

The rest of the car is just as reliable as anything else - the later (Rover Longbridge) built cars are better screwed together than the early (BMW Cowley) models. you can tell if it's a Rover built car by the body coloured sill. That said the "BMW built" ones were stll well put together.

Automatics use the ubiquitous Jatco 'box, not the longest lasting 'boxes in the world but can be replaced for about £600.