E30 has PCD 100 holes which is one of the 2 common PCDs, the other being 114.3. So PCD won't be the biggest issue especially for those jap rims with 8 holes to fit both 100 and 114.3. At least not like the Alfa which uses PCD 98, lagi teruk.
for a 15x7j rims, the ideal offset is between 20-25. As for the hub core, you have got to measure. in other words, if you have a 15x7 rims with offset being 35 or higher, you would need to made a spacer to cater for the difference. For example, if the rim's offset is 35, u need a 10mm spacer. if 40, u need a 15 mm spacer..
I'm giving a benchmark based on 15x7 rim.. So it may not apply exactly to say 16x7.5 or 15x6.5.... However, it should be about the same range...