Here's some info on the 3-Series. Model-wise, all 318's have a 1.8L (later 1.9L) inline-4 engine making 138 horsepower. The 325's have a 2.5L inline-6 making 189 hp. 328's replaced the 325's and have a larger 2.8L inline-6 making 190 hp. M3's have a 3.0L (later 3.2L) inline-6 making 240 hp. What follows is a brief history of the E36.
1992: the E36 3-Series is introduced with the 318i and 325i sedans.
1993: the E36 318is and 325is coupes are introduced
1994: the E36 318i and 325i convertibles replace the old E30 versions; a passenger-side air bag is added to all 3-Series models; ASC (All Season Traction) becomes on option
1995: the BMW Motorsport tuned M3 coupe becomes available
1996: the 328i sedan and 328is coupe replace the 325i and 325is. The new cars have a stroked 2.8L version of the 2.5L engine. Horsepower increases by 1 from 189 to 190, but torque jumps from 181 lb.-ft. to 207 lb.-ft. The M3 gets a larger 3.2L inline-6 making the same horsepower but more torque
1997: An M3 sedan is introduced
1998: last year for the E36 sedans and all of the 318 models; the 323is coupe and 323i convertible replace the 4-cylinder in the 318is and 318i convertible with a 170 hp 2.5L inline-6; the M3 convertible is introduced
1999: the first E46 sedans (323i and 328i) go on sale; this is the last year for the E36 coupes and convertibles and the E36 M3
As you've probably now noticed an 'i' (as in 325i) means the car is a sedan. 'is' (as in 325is) means the car is a coupe. Sometimes you'll see a 'C' after the designation (as in 325iC). This denotes a convertible, but the 'C' is not actually included in the script on the trunklid.
The 318ti is a 4-cylinder hatchback version of the 3-Series, introduced about 1995 or 1996 and running through 1999. Scoffed at by many BMW enthusiasts as not being a 'real BMW', it nevertheless possesses many 3-Series virtues and adds a measure of practicality since no E36 wagons were ever available in the U.S.
Unless you are concerned about fuel economy, I'd recommend you choose a 325 or 328 over a 318. The six-cylinder engines in these cars are at least as dependable as the fours in the 318's, and are faster and more popular. A 325 won't really carry much of a price premium over a 318. However, there's nothing wrong with a 318 should you choose to get one instead. An M3 is a great choice too if you can afford one--no major problems I'm aware of, but you should check out other E36 enthusiast sites before purchasing.
All the above cars (except M3 coupes--5 spd only) were available with either manual or automatic transmissions in any given model year.
Here are some sites with aftermarket equipment for BMW's in general:
www.bavarianautosport.com (all sorts of parts, accessories, performance upgrades, etc.),
www.dinanbmw.com (performance),
www.autothority.com (performance). Also check out BMP Design at
www.bmpd.com (parts, accessories, performance upgrades).
Keep in mind that the E36 may well be the most popular BMW in recent history. All manner of other resources exist.
Good luck--hope you find this helpful (and not too long-winded)