Tim, please allow me a few corrections:

1) Bank acct: for non-residents, these days only a few banks offer it. International ones mostly like Deutsche Bank for instance.

2) NIE: you can ask for a NIE without becoming a resident, yes. But for that you must show that you really have a legit need for it without becoming resident. In most cases it's easier and more appropriate to ask just for a NIF, a fiscal number.

3) Fiscal residency: there are aspects other than only the 183 days rule. Most important is having in Spain its personal base of affairs. This is something which the fiscal administration may decide by itself and in order to contradict them is a pain on this particular item. If you have a house in Spain and come back between assignments as the original poster said, there's no way around this rule.

4) Driving license: for EU nationals you must change it for a Spanish one during the first two years after obtaining your residency or alternatively follow a process to have your original license registered in the Spanish database and a credit of points which in the end is harder than changing it outright. The change of the license is a straight-forward procedure and the time it takes depends more on the time taken by the country which issues the driving license to confirm its validity rather than anything else. For nationals from a country other than an EU country it depends on a country-by-country basis. As a general rule the license must be changed after six months residence. For nationals of EU countries, not changing the license in the two years after obtaining residency subjects them to a 500€ fine. For all others it is a criminal offense for the original license is deemed invalid and therefore is the same as driving without a license.

5) Importing cars or motorcycles: It's not an hassle to import a car or bike. The whole process can be done in 3-4 days. It's expensive, yes, although cheaper than in Portugal. But not difficult.