I would choose another 318, the bonus being that you can swap parts if one of them is acting up or dead. The eta 5er is a dog (you may or may not care), and the E36's, like you mention, have cooling system maladies, and I personally believe the build quality is just not as good as the E30. I had an E36 325 before, and while the engine is super smooth, I'll never look back now that I've gone E30.
If you'd like something a little larger, the only later model car I can recommend that'll meet your fuel economy requirements would be an e39 328i. It'd probably be about double the cost of the other cars you've mentioned, but worth it! Or what about an e34 525, those are nice too. Captains chairs ROCK for longer drives.
And yes, 5-spd will always be more reliable than auto. And more fun to drive! The autos in the 5's and 7's are ok, but not so hot in the 3's.
Buy a nice 5 and retire your 'vert to me. I promise to take good care of it.
