Yeah 5 speed sport models and 5 speed cali-tops are hard enough to find, especially ones in good condition that haven't been molested. I'm sure you'll enjoy this car for many years to come. Best wishes!
BTW, there's plenty of good DIY/tutorials and tech articles posted in this sites Knowledge Base section and many of our members are happy to offer help and advice. Here's a few other websites that you'll find useful as well:
www.realoem.com (great site for looking up part numbers and diagrams, just punch in the last 7 digits of your VIN and a complete list of parts for your vehicle will appear)
www.getbmwparts.com (Tischer BMW sells Genuine BMW parts for a fraction of what the dealerships charge. Shipping is reasonably fast and their phone support and customer service is top notch. They're very friendly to the 318ti community, often giving us slightly better deals than ordinary BMW owners)
www.pelicanparts.com (I usually compare this sites prices against Tischer's before purchasing parts. It's hit and miss, but sometimes there's a significant difference between the two. Pelican offers free shipping on orders over 75 bucks. Shipping is usually pretty fast)
www.ddmtuning.com (Their shipping is slow and customer service sucks, but they have the best prices on headlight packages and Mtech front bumpers) A common upgrade for these cars is euro projector headlights with glass lenses and HID bulbs. There's only two brands of headlights that I'd recommend. ZKW makes the best, but they're really expensive, beware of sellers calling their lights "ZKW style", they're either ZKW's or they're not. The other brand is Depo Gen4's, they look just like ZKW's and perform well for the price. DDM sells kits for about 130 bucks.
Besides that, another thing to remember, the 318ti is the same as the e36 sedan from the "A" pillars forward. This includes the mirrors, fenders and hood, with the exception of the nose panel and front bumper which is interchangeable with e36 coupes, sedans, convertibles and compacts. Everything aft of the "A" pillars is 318ti specific.
You can use e36 coupe front seats since they're a direct fit, however the seat belt receptacle may need to be swapped depending on what year they were installed on. (When swapping receptacles, it's very important to disarm the tensioner before removing or handling them) Sedan front seats, while they will bolt up, aren't recommended because they don't tilt forward to allow passengers in the back seat. Rear seats are Ti specific, coupe and sedan rear seats won't fit.
Also note, the front seat belt receptacles are armed with an Automatic Locking Tensioner. This device retracts the recepticle about 2 inches in the event of an accident. 318ti's built up to 03/97 use a mechanical auto-locking mechanism(part#'s 72118167711 left and 72118167712 right) while models built after 03/97 use a pyrotechnic auto-locking mechanism (part#'s 72118221013 left and 72118221014 right). The Ti uses a semi trailing arm rear suspension similar to e30's and Z3's, while e36 coupe sedans and verts use a multi-link design, so rear suspension components are not interchangeable. Front suspension components are however, just make sure the spring rates are correct ie; don't install 6cyl springs on a 4cyl model or you'll end up with reverse rake.
Most common problem with these cars is the window regulator sliders. The windows are known to come off track and fall into the door. I wrote a nice tutorial and posted it in the knowledge base section of the forum.
The leading cause of death fo these cars is head gasket failure caused by faulty cooling systems. Most of these cars have over 100k miles on thew clocks and the plastic cooling system components have become brittle. There's two plastic coolant fittings inparticular that are know to break causing all of the coolant to dump instantly. I highly recommend replacing the entire cooling system for peace of mind. It can be done over a weekend for under 400 bucks and there's a great writeup called "Cooling System Rebuild" in the knowledge base section.
This next bit of information is very important. NEVER, I repeat NEVER let your engine overheat. If you notice that needle moving, shut it off immediately! I don't care if your house or a gas station is only a couple blocks away, shut it off and coast if you have to, but don't let it overheat. If it overheats for even a second, you run a huge risk of blowing the head gasket or at least weakening it significantly. Unless you have an extra 2 grand burning a hole in your pocket, heed my warning.
And finally, if you notice the heater isn't blowing hot air, you probably need to bleed the air out of the radiator. These engines like to trap air in the cooling system. Sometimes it's caused by a cracked radiator or a leaking fitting, whatever the case, if air builds up in the system it will create whats called air-lock. Basically an air bubble forms in the radiator and prevents coolant from circulating through the engine and heater core, thus little or no heat coming from the heater. It also means no coolant submeging the temperature sensor, thus fan not working and possibly damaging the sensor. So if you notice your heater not putting out hot air(and it should burn your hand if you have it on full blast), you probably need to bleed your cooling system. There's a good writeup called "how to properly bleed your cooling system" in the knowledge base section.
Whew... That was a mouthfull. Sorry for the long post, I just want to do my part in keeping a semi-rare car on the road. 5 speed cali-tops are kinda special IMO. Please don't do anything dumb like gut your cats or trick your o2 sensors. Kids do this crap all the time to these cars and it pisses me off. That's why it's so hard to find a decent ti these days. There's no horsepower gains to be had with these little engines unless you want to install a supercharger or do a 6cyl swap. Best bang for the buck is sport tuned suspension and sticky tires. Or you can always just enjoy it for the nimble/reliable/economical hatchback that it is
Best of luck to ya, don't hesitate to hit me up if you have any questions.