I've owned two manta GT/e's both coupe's both standard engines, both had covered over 100,000 miles and would still do 120mph (on a private test track that is!, the M3 I think it was called!)
My second Coupe A 1986/D reg had covered 155,000 miles when it was to rotten for another MOT.The car was thrashed around goodwood a few months before having the thrashing of a lifetime in one day and took everything I threw at it.
The way to go today engine wise seems to be the good old 2.016v xe or the rover V8.

The opel manta is a very good reliable car which needs to be talked about more the car has many different engiens and to shapes the hatch/coupe there are different engine sizes 1.6 / 1.8 / 2.0 and are very easy to modify under the engine and the body.