BMW E36 Buyers Guide and Common Problems:

BMW E36 3-Series, Production 1991-1999.

Body:

Mk1 from 1991-1996
Mk2 from 1996-1999

Difference between Mk1 and Mk2:
Nosecone and grills. (Pre-Facelift grills are skinier)
Wings are different, (Larger hole for Indictors)
Wing Indicators (Pre-Facelift Indicators have a visible screw in them and have a black plastic around them)
Clusters (Pre-Facelift go to 6k before redline, Facelift go to 6.5k before redline)

Bodystyles:
Saloon
Coupe
Touring
Compact

Coupe:
Same wings, front indicators, bonnet, front doors & rear lights as the Convertible. Boot is unique.
Convertible:
Same wings, front indicators, bonnet, front doors & rear lights as the Coupe. Boot is unique.
Saloon:
Same wings, front indicators & bonnet as the Compact & Touring and same front doors as the Touring. Rear doors, rear lights and boot are unique.
Touring:
Same wings, front indicators & bonnet as the Saloon & Compact and same front doors as the Saloon. Rear doors, rear lights and tailgate are unique.
Compact:
Same wings & bonnet as the Saloon & Touring. Doors, rear lights & tailgate are unique.

The headlights fit all models but do require wiring modification for use in the Compact

Front seats will also fit all models, Rear seats are all unique.

Engines:

Engines 4 Cylinders:
316i SOHC
1990-1993 M40B16 99hp
1993-1999 M43B16 101hp

318i SOHC
1990-1993 M40B18 111hp
1993-1999 M43B18 113hp

318is DOHC
1992-1995 M42B18 138hp
1995-1999 M44B19 138hp

Engines 6 Cylinders:
320i DOHC
1991-1994 M50B20 148hp
1994-1998 M52B20 148hp

323i DOHC
1995-1998 M52B25 168hp

325i DOHC
1991-1999 M50B25 189hp

328i DOHC
1995-1999 M52B28 190hp

Engines Diesel:
318tds
1994-1999 M41D17 89hp

325td
1991-1999 M51D25 113hp

325tds
1993-1999 M51D25 141hp

Engines M3:
M3:
1992-1995 3.0 S50B30 286hp

M3 EVO:
1995-1999 3.2 S50B32 317hp

Common Problems:

Rust:
I'll just post the post common places:

Rear Arches and front arches,
Lower front wings,
Boot Door, (Under Spoiler, Around lock, and bottom corners also check for crack near latch)
Rear Pannel, (Under the boot corners)
Jacking Points, (Both underneath and on the outer sill)
Spare Wheel well and Battery corner (In 6 cylinders)
Rear Shock Towers,

Vanos Unit:
Only on 6 cylinders, You will hear it if it's not right. Usually a loud ticking noise. (Google Dr Vanos for repair)

Electric Windows:
Check the go up and down smoothly,
Verts/Coupes have a drop window function, If it doesn't work. The door catch is worn. (About €42 each from BMW and only 2 torx to fit)
Window regulators are riveted so can be a pain to change, (When I replaced mine, I was able to use nuts and bolts)

Waterpumps:
Originals are plastic impellers, And should be replaced with metal Impellers (Most common fault of E36 Deaths is Headgasket trouble as a result of Waterpump failure)

Timing Belt or Chain?
Nearly all E36s are Chain Driven and realistically should never need changing, However some of the very earlier 316 and 318 do have a Timing Belt.

Running Rough?
Possibly a Vanos issue,
Idle Control Valve, (Just remove it and clean it)
Air flow Meter,
Or possilby Lambda sensor,

Nikasil Issue
The 323i (M52b25)
Expierience cylinder bore wear, resulting in a loss of power and compression, Low quality petrol will wear it quicker.

To check which you have, look under the car under the starter motor, the steel lined engines have a raised steel tab and the Nikasil ones have an indented one.

Wiring Harness
A very common problem, Is the wiring loom in the boot. On the left near the hinge. Check that it's not worn!

Climate Control
If it doesn't work, It's possibly a very simple fix.
A small capicitor inside the unit (Costs about .50c to get a new one. Google for DIY threads)

All parts can easily be sourced easily from breakers or from a local Motor Factors or in some cases BMW themselves are just as cheap.

VIN Number Locations::
These can be located in various places:
Passenger corner of windscreen,
Sticker behind rear number plate,
Sticker on shock tower under bonnet,
Stamped into the windscreen panel on driver side, (Need to have bonnet open)

SE or M-Tech:
These are the 2 body-style options. (There's also an Aero option but that is very rare)

What's the difference?

Well an SE mainly basic, Cloth seats, Twisty knobs instead of air-con, Basic electrics and comfort seats.

An M-Tech or M-Sport is usually a lot better speced, They have the same bumpers and side skirts as an M3, Side trim mouldings are also the same. Sport seats in either leather or half leather, Climate Control, and a black headlining.

Obviously check the usual also:
Get a History report, (Make sure it covers UK history incase of import)
Crash Damage,
Weird noises/knocks when driving,
Service History,
Etc...

You can check the factory spec of an E36 here using the last 7 digits of the VIN:
http://www.bmwvin.com

This is all from my own personal experience with the E36, Engine Data I'v got from Wikipedia.
If there's anything I have missed or got wrong or if someone wants to do a piece on the mechanical problems, Please PM me and I'll update this post.