Hi I would be looking for absolute min of 1.5 times thread dia ie 18mm of thread available. This equals 12 turns of the wheel bolt. If not longer bolts for peace of mind?
Yep ET33 as already said
There are spacers, purpose-designed specifically for your car, which bolt on to your hub and fit centrally over your hub centre spigot as per original wheel; in turn they provide a male spigot for your wheel to sit on, just like original; I guess these are not what you have as they are normally at least 15mm thick and are supplied with extended bolts.
Then there are spacers which are just unattached shims, typically available in the range 3-19mm. No central support for your wheel, so all of the load in shear is taken by the bolts, which was not the designer's intentions; this is made worse by the fact there is now a 5mm or whatever gap between hub and wheel. Also you can get imbalance problems as the wheel is now only maybe centred on the hub. yeah I've run them in the past, but now I think they're a VERY BAD IDEA, just IMHO, sorry if that doesn't help...........