I used to think I could stick a 16v engine in my old 1.2 Clio. Just have a careful look at the complexity of it all - diffs, gbox, brakes, suspension, electrics, cooling, starting, charging etc.
If youre going to get a mechanically perfect 172 for silly money and youre going to swap every last part, then fair enough. But it isnt as simple as dropping another lump in.
Id say £5k minimum, plus cost of your car.