1) front and rear ET=23.3 mm. At the rear an ET of 15 mm will still fit
2) Normal is 195/65/15, 215/60/15 was an option. 225 front and 255 rear will
fit.
3)

15 inch    195/65/15, 205/60/15, 215/60/15 and 225/55/15
16 inch    205/55/16, 225/50/16 and 245/45/16
17 inch    205/50/17, 225/45/17 and 255/40/17

Porsche company does niet allow 18 inch on 944 models. For 17 inch you need
to get some new parts for  the camber change 17 inch requires officially. I
don't do this and my 17 inch front tires hit something on maximum steering.

Cheapest change is going to 16 inch early turbo rims. Otherwise try to find
17 inch 911 rims with an offset of 23.3 to avoid the need of spacers. I have
bought boxter rims for my 87 944 but those rims won't fit on a early 944
without spacers.

Have fun, Paul