1. how is it with parts sharing among 145 and 33. I've heard about the clutch. Anything else? (the point is it might be cheaper for a 145/146)
If we are talking for Boxer 145/146 yes they share almost 80% of parts including gearbox. Slight difference is cams (milder) and on electronic ignitions and sensors due to emisions. Same reason different (uneven ) design on exhaust manifold but can be installed on 33 but only from 8v to 8v. 16v doesnt fit.
2. brake upgrading. I'm not looking for anything insane. However do you think wented 239 mm will do? Are they standard on 1.7 8v? Will 145 257 mm fit?
The cheapest upgrade you can have is vented front disks from 33 16v and if you find , rear axle with disks too . But front vented disks are good which can be combined with braided pipes.
3. I've heard anything about a lightened flywheel? What is it good for and where to get it?
No need for that , leave it as it is. This is useful for racing purpose otherwise you will loose torque. If you want to spent money , spent them on quife differencial although this is rare to find.
4. What about exhaust manifolds? Is it worth to think about anything like this? - Alfa Romeo Shop - Tuning, Styling, Fanartikel und Modellautos - Alfa 33 CSC Sport Fcherkrmmer CSC Fcherkrmmer 4-2-1 31954
Probably the best design but not cheap..
5. what 14" rims option are there?
You can use rims that can be fitted on Fiat and Lancias also.Better go for for 15'' as you will have larger vriety of tyres and cheaper .
6. is it possible to get those P4 side skirts in aftermarket?
Probably yes , i am not sure....
7. is a P4 conversion possible on 1.7 8v engine? How difficult it is?
Yes you can but... You need to modify rear boot floor in order to accommodate rear differential and probably a gas tanks from p4 model which means lot of welding. Also , consumption will rise significally...
8. Anything interesting about the interior? I've seen those lovely Recaro seats and door panels? Can I hope to get them somewhere?
Yes , 33s , especially 16v had recaro interios. Also . from same era had similar seats also Fiestas and other for models which had identical pattern on fabric. With little luck and small modifications will probably find recaros. Other wise go for recaros from lancia delta. Also very good although no much with the fabric.