Whatever you do dont buy an auto, dont waste your time, you will end up selling the car to buy a 5 speed sooner or later if you do.
E30's can be reliable and can be had extremely cheap depending on what you want.
Eta's are great cars for the money - good on gas, good torque off the line, they just dont have shit up top. For the cash they are great starter bimmers.. they can be had for as little as 1000. I would never pay more than 2k for an eta though.
E30 325i/is are very nice, and will hang with an e36 325 for a while (I think they start to fall behind through 4th gear (0-60 is almost identical)) Great cars for the money - The IS is worth waiting for, as it has an LSD, sport seats, and alot of other random goodies. An is can range from 2000 for an early metal bumper model, to 4500 for an immaculate late model plastic bumper car.
E36 325's are great cars. They handle great, they are 5 lug unlike the e30, they have extremely modern interiors considering how old they are, and best of all.. 24v sexiness. E36 325's are coming WAYY down in price these days. I see 100k mile 325's go for as little as 5 grand. 150k miles e36's can go for as little as 4k - e36's with this kind of mileage are great, most people buying e36's for the pimp factor are scared away by the higher mileage which makes them go down in price, while still being very reliable. I would not doubt the m50 in terms of longevity. Also, 4 doors go cheaper than 2 doors most of the time, while the cars are identical performance wise.
With the e36, the one thing that you have to be aware of is the cooling system, if it hasnt been dealt with by the PO then be prepared to drop 1k after buying the car. Waterpump, thermostat, thermostat housing, hoses, and radiator are all items that fail on the e36- as you can see that is the entire cooling system.
Just keep your eyes peeled and check every car site everyday, and you will be able to pic up a nice 100-130k mile e36 325 for 4500-5500 bucks. Dont be afraid to buy something with some issues either, its possible to buy an e36 in need of new brakes, or other random shit for 2500-3000 and then just dump 1 grand into it and have a very nice daily.