IMO...I agree with the advice given by most of the sifus here. I understand your concern of not wanting to get a different car which will be considered as second hand compard to the car your family has owned from day one. You also need to consider that there are many very well kept one owner, low milage and BARGAIN priced 328 in the market looking for a new owner. The upholstery and build of the 328 is very much better than the 318. Check them out and you will be surprise.
One more point is that if your engine transplant is not done well or the transplant engine is not up to par, you may have to deal with the hassle of frequent visits to your mech to rectify these problems, which will make your car become very like the 'second hand' car you try to avoid.
BTW IIRC our youngest member is 21. can you beat that?