There is a correlation that Switzerland and Norway are outside the EU yet they have decent economies, but that doesn't mean there is causation.
There are other countries outside of the EU that aren't doing so well.
Switzerland has been a haven for investors for a long time, due to its political neutrality (it didn't have a costly World War to fund) and stability.
Norway has the biggest oil wealth fund in the world.
Both have relatively small populations.
It's often worth noting that the bulk of Norways jobs are in the public sector and their manufacturing output isn't particularly great.
Switzerland in particularly did negotiate something like 120 individual agreements which took 3 years. My guess is that it might take a bit longer to sort ours out if we decide to leave being that we are about 10 times bigger and without a healthy bank balance.
Why would businesses not invest? Investors like certainty and stability. Leaving the EU serves neither so why would they invest here instead of say Germany? On the other hand, its been great that we haven't been in the Euro for the same reasons.
EU Laws - If you trade with the EU, then you need to play by its laws if you want to trade with it. Many of the laws make sense in my view anyway but you can find problems if you cherrypick, as you would do with normal UK laws. The US government recently released a report saying that they may have to look at their close relationship with us, as they want us having our say at the EU table. So this idea that we can just work with the US, is not perhaps the view of the US.
Spain - Potentially, who knows what would happen if the politics went a bit sour.