You need about 25 - 30mm. Opinions vary, but I think that at that thickness, you are better off going with adapters, not spacers and long studs. Make sure the adapters are both hubcentric and wheelcentric - i.e. the center bores interact properly with the car hub and the wheel to keep things centered properly.
As for the safety aspect? Properly made adapters (that's a big caveat, BTW) should be as strong as a wheel is in the first place. There's no more stress on an adapter than there is on the hub of an alloy wheel bolted to a hub.
I'd look for a good alloy, and press in studs. Wouldn't want anything with threaded studs in aluminum.
I run 25mm adapters in front, 32 adapters in back and now can put any of the much more common FWD offset wheels on my 240. I currently have some 17X7 Sport Edition D3's, which were taken off a Jaguar S-Type (I bought them nearly new on eBay).
Here you can see the 32mm adapters in back:


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'63 PV544 rat rod, '93 Classic #1141 245 +t
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