So, welcome Richard, let's see if I can provide some info...
A 318ti is quite a simple car to maintain, especially if you have friends/contacts close by. The Ti is a bit different from the other E36's (Coupe, Wagon, Sedan) so not all parts can be interchanged but many can. They're pretty simple cars so very little specialist BMW knowledge would be required. If you're happy to approach the guys around the block then it might pay to have a chat with them before you purchase to get any specific tips etc.
The "rough idle" is probably a vacuum leak which is a common occurance on these cars, your family friend that owns an auto shop should be able to do a smoke test on the car to identify any leaks. Could be as simple as replacing an inexpensive hose.
Not sure about the power steering, depends what wrong with it. If it need a new rack that'll be most expensive (but I have one available for $150 if you need one). Get your auto shop guy to do a pre-purchase inspection and advise you on that also.
They don't use a lot of fuel but it's still a 20 year old car and might be a little inefficient. The rough idle won't help but a $100 tank of fuel will last you ~500km's I'd think.
Definitely start with a cheap car first! The insurance will be cheaper, you won't care if somebody (or you) dings it, curbed wheels aren't such an issue etc, etc. Learn to drive well before getting a good car.
A 206xxx km, 1998 E36 318Ti would be an ideal first car I'd say... especially if you can get it for $1200 and your mechanic family friend checks it out first and says it's okay.
Good luck and keep us posted!