I learned to drive in a W124 300D auto. It was a super car, big, possibly the safest benz ever, and fast enough for a learner, though to be sure it was no road burner. It was also extremely reliable, which I think is important, especially for a younger girl.
Having passed my test I bought a 1984 BMW 316, which had a 1800cc carburated 4 cylinder engine, 75bhp 100lb/ft torque (M10-good for 1,500 bhp in F1 trim). It cost me £1,250 new and I sold it 4 years later for £750 with no problems whatsoever in between. A fine car for a new driver: kept up with the traffic, had BMW prestige, safer than a compact, and very reliable. So I would second Davis Lo's suggested E36 318i SE.
I had access to an E36 316i coupe for a long time. Again, no road racer, but it was fast enough to pull out of motorway junctions and so on. What I liked was the very communicative chassis and steering and the beautiful gearchange. Not fast, bu t it made you feel like a great driver.
My own view is that the W201 is one of the finest Mercedes saloons ever built. Long-lived, discreet and a triumph of function over form. And very very safe. A petrol is bound to be better, perhaps a 190E 1.8 or 2.0? If you can get one bought new by a grandad about 10 years ago...