Great idea, just do it! I have a 1.7 ie motor in my Alfasud and it was the best thing I have done.
My only suggestion is to be careful of secondhand motors, if you are paying somebody else to do the conversion it could work out very expensive if the motor turned out to be worn out (as most of them are by now)
Cost it out carefully and make sure you buy the engine from a reputable supplier so you have some backup. It is often cheaper to buy another whole car, at least this way you have the opportunity to drive the engine in the car and get a much better idea of condition.
My personal preference is for the injected engines which are less powerful and more difficult to fit, but are more economical and last longer than the carburetted engines, either way- Go for it!