Sounds like you're doing great maintnance, however, replacing your spark plugs about 3-4k miles is really not needed. Most spark plugs are good for 25k if you have a strong running engine without leaks or burning oil or anything. I personally would fill your oil with Mobil 1 0W-40. I used to use Pennzoil 5W-50 but they stopped making it. And yes, absolutely replace your fuel filter. That should be done at least every 25k. I do it along with my plugs. Let's see, 92k your belt tensioners should be getting old. But they might be good for a while, either way, they're about 60 dollars a piece so keep an eye on them, nothing worse than having krickets under your hood. Mine personally lasted to about 125k. But you'll have to know whether or not you have a 70 amp or an 80 amp alternator. They're specific to each alternator. As far as the belts, I don't really know the setup in your engine. I have a bently manual, I just don't have it with me haha, so I don't know what you have. Now I don't know about the 318i but in the 325 BMW got kinda cheap in a couple places. They put a plastic water pump in the engine. This should be the VERY FIRST thing you replace. Go to
www.thepartsbin.com and order a new water pump with a metal impellar. If in fact they gave you a plastic one. Also, your thermostat housing is most likely plastic, replace that also with an aluminum one from The Parts Bin. While you're at it replace your thermostat with a colder one. The Parts Bin offers an 88 degree. Nothin better than making sure your engine doesn't overheat. Now here is where you're going to save a lot of money. Buy a friggin O2 sensor. You have 92k on your car, obviously it's never been replaced (BMW won't do it untill 100k) If you run your car with a bad O2 sensor too long, you will destroy your catalytic converter (believe me, I did) and cats are expensive. So according to The Parts Bin...
Water Pump with a metal impellar $54.03
Water Pump O-Ring $3.93
If it's not already aluminum
Thermostat Housing (Aluminum) $34.28
Thermostat Housing (Gasket) $5.10
Thermostat (88 Degree) $23.82
Thermostat O-Ring $1.20
Oxygen Sensor $93.60
Belt Tensioner (either 70 or 80) $60.00
Fuel Filter $14.92
Total $290.88
I know that's over your price range but not all of this has to be done at once. If I were you, I'd start with the cooling system stuff. The O2 sensor is the last thing you can replace, but just make sure you don't wait until your check engine light comes on. Also, in case you haven't realized already, your bentley manual will explain your On Board Diagnostic (OBD) system. It's SUPER convenient, especially when you're getting your check engine light on. I wish I could help more but I don't know much about the M42 engine, I know a LOT about the M50 and S50 engines though haha.