Have I finally convinced you? yes I think you should do it- a Sprint 1.7ie would be a really nice combination, it is what I originally intended to do before I got my Sud...
In terms of what injected engine to use there is only one option in Australia which is the 1.7ie with Le3.2 Jetronic fuel injection which was sold between late 1988 and 1990. All later 33's imported were 16 Valve which is a harder and more expensive conversion.
The answer to your gearbox and exhaust questions depends on what model of Sprint you have and how much work you want to do! The easiest sprint to convert is the 33 based model (with outboard front brakes and rear drums) if you have this model you can bolt in any 33 gearbox (although you will have to adapt the Alfasud speedo drive as the 33 uses an electronic pickup in the gearbox instead of a worm gear arrangement)
If you have the earlier Alfasud based car with inboard discs (as I do) you will have less choices. The Alfasud gearbox casing is different as the brakes bolt on to the sides of it. You can fit the brake calipers to a 33 box but it involves accurately drilling and tapping an internal thread into the housing to bolt the calipers on. This is possible but it is usually easier to rebuild 33 internals into am Alfasud casing- not a 5 minute job though... I have just used the standard Alfasud gearbox and haven't had any problems in 2yrs of use but it isn't as strong as the 33 gearbox.
If you have the 33 brakes you can use any 33 exhaust- you can use the standard 33 setup if you wish although I can't imagine that fitting the catalytic convertor will be high on your list of priorities. You can't use the Alfa 33 exhaust on an Alfasud based car as it doesn't clear the front brake discs. To start with you can use the standard 1.5 twin carb exhaust, it will bolt up to the 1.7ie motor without modification. I have the 1.5 exhaust which works ok although it strangles the top end a bit it gives great low down torque. If you want to compare the standard 1.7ie setup with modified 1.5 and 1.7 engine cars have a look at the dyno results on www.alfaclubvic.org.au
You can see why my next modification is the exhaust!
In theory (Australian Law) the emission control equipment must be present for the legislation present at the date of engine manufacture. I.e. for a 33 1.7ie it should have a catalytic convertor fitted, a charcoal cannister for evaporative emissions, an oxygen sensor and unleaded fuel filler (restricted to allow only unleaded nozzles to fit)
However if you were to fit the injection system to your "standard" engine then the vehicle would only have to comply with legislation current at the date of first registration. If the engine number was the same as the original, the injection is legal without additional emission control- should your vehicle be tested it will easily pass any emission tests relevant to the carburetted car.
Obviously your insurance company needs to know- I advised mine and they had no problems with the conversion. It probably helps to be over 25 though...
I will keep a look out for a cheap or damaged 1.7ie for you, Of course the best way to be convinced is to have a look and a drive! If you need some encouragement,let me know next when you are coming up to Melbourne and we can find some good roads that might just help you make up your mind....