8" will fit, so long as the offset is correct. it will need to be around +35mm, give/take 3mm.
225/40 is the appropriate tyre for this wheel on that car.
as above, bigger wheels are a compromise, but that's ok for many people. it will offer a 'harder' ride, though is subject to tyre choice (some definitely have stiffer tyre walls, like high-end Bridgestone) and pressures.
i personally buy a high quality tyre, of which there's heaps to choose from. my personal shortlist would include Pirelli PZero, Bridgestone RE001 or RE050, Goodyear F1 Eagle or Asymmetric. i have used 'better value' brands too, and have no probs recommending Falken and Toyo.
for better ride comfort and handling, with bigger (heavier) wheels, it is worth upgrading the dampers too, even if it's in the future.
with 17s, you could go with 215/45 or 225/45 tyres.
alfa bolt pattern is uncommon: 5x98mm. so wheel choice - if you want perfect fitment - is limited. it is easiest to buy original alfa wheels (or copies) for direct fitment. there's obviously a big range, because you can choose from any that are designed for the 147, 156, and GT.
with non-alfa wheels, you may need to buy 5x100mm wheels. then use 'wobble bolts' which can hold them in position. that is safe and common practice. but you must ensure the centre bore hole (the big hole in the centre) has the right spacer ring fitted to provide a 58.1mm diameter hole, so that the wheel fits snuggly onto the alfa hub. the hub takes most of the force/weight; the bolts just help hold the wheel in position onto the hub.