|
E39 M5 will probably be easier to source parts, they are awesome cars.
E31, I have driven and maintained an 850i auto. Has steering box which felt more loose to me, kinda floaty even for a BMW. Not really fast, the E39 M5 felt much faster, revved more freely. Might be a long way off from a Z4 in terms of handing, weight, and quickness.
The M5 also has a steering box but the one I drove was very tight even at 100k miles, unlike the E31 with less miles, might be car specific according to wear and mileage.
I would probably recommend M5 overall, manual E31 would be fun for sure and maybe better investment due to rarity? I recommend test driving one first, I found the E31 to be more "vintage" in terms of handling and power, where the E39 M5 felt modern but still exciting.
Good luck, have fun!!
|