Hi, FMD159.
I've just spent an enjoyable afternoon with a large mug of Bustello coffee reading through the comments here. First of all, congratulations on getting your Alfa. It sounds like a very interesting car. You are fortunate to have very knowledgable bunch of Alfa enthusiasts in OZ. That can make owning an Alfa a lot more fun.
There's a lot of good advice here and so I'll try not to repeat anything that's already been said. I'd like to make a couple of suggestions, however.
First of all, whatever you do, just wait a bit. Get to know some of the people in the Alfa community, go see some cars, talk to people working on building cars. Once you do this you'll be surprised at how different you think about your car.
Re: the motor change. The motor in your car is, in all respects, a much more enjoyable motor to drive than the 2L motor you want to replace it with. It's a far more responsive motor. Also, in day to day driving, a well tuned 1750 GTV will not be left behind a by 2.0 GTV. You may be a little behind in places but, trust me, you'll be having a far better ride despite that. The 1750 GTV is generally regarded by most authorities as the best of all the various iterations of GTV. Period.
Having said this, I am also a firm believer in "period modifications". Alfa has a long and storied history of building factory hot-rods. Personally, I don't think there's anything wrong with making changes that are in the spirit of the kinds of things the factory guys and various tuners were doing in period. For instance, my car is going to get a Alfa close ratio gear box, 4:56 LSD, and a much modified 2.0 motor. I realise this is heresy to originality advocates, but it's the kind of car I've wanted to build for a long time. I don't plan to do anything to the car (fender flares, etc.) that can't be put back to original with relative ease. All the 1750 specific parts (like the motor, etc.) are going to be carefully identified and stored so that they can be passed on to the next owner to use in the car's restoration.
But the bottom line is this: the car is yours, you can do what you please with it. But, please keep in mind that you are definitely "preachin' to the choir" here.
I recommend that you do the following. 1. Get the car running, 2. Fix the brakes, 3. Get it clean inside and out, 4. Spend a couple of days compounding the paint. Alfa's original factory paint is enamel and is quite durable---even after 20 years. You can even lightly sand it with fine grade paper and then compound the surface. You may find that the color is quite fitting for the car.
In general, however, get the know the car and get to know the people who like these cars. The learning cruve isn't that steep and, judging from the people who post here from OZ, they're a pretty good bunch of folks.
Good luck.