Any car no matter how much HP can go fast enough in a straight line to get you speeding tickets and loss of alot of $$$.
So better to think of it this way, how many places in the united states does the straight line speed really limit you? Its plain stupid to drag people at lights, if you are doing that prepare to lose your DL within a few years.
How many places does the top speed not besting 125 really matter? well most tracks aren't big enough for that to be a huge concern in street cars, and on most any highway such excessive speed will get you in an accident or loss of DL... so same deal.
So what matters? The e30 318's handle extremely well, they are a joy to toss around even if the curvy roads near you are only 25 mph turns.
Learn to drive the car that being, with a 318 you want to get it above 3000, preferably 3500 and keep it there. Having driven Porsche 356's which have a similar powerband (but foul out their plugs if you try to accelerate at under 2k rpm or don't get them cleared out) this comes naturally, but you'd be amazed how many people don't understand why the far side of 6K rpm is an acceptable way to drive. Learn to downshift your car at speed to keep it in its powerband, and learn to get into first gear when you can, its amazing how some people cannot shift into first until they are coming to a stop, but if you can get your downshift into 1st gear you will have all the acceleration you need.
Best updates to either the 318 or 325 will be good tires, good brakes, and if the suspension bushing haven't been redone you will be doing this.
It is often cheaper to find a car that has had struts, shocks, bushings etc. sorted out. Because unless you have done this sort of thing before you'd do well to have a professional do it. Bushings often require access to a shop press, struts, tie rod bushings, and some bushings will require an alignment afterwards.
So where as you can find these cars for 200 bucks in bad shape, by the time you get all that sorted you might as well pay more and get a car that's not a basket case.