Upload & Sell: Off
600GB is a pretty reasonable amount and not difficult to manage because you can fit the entire collection on single drives. Therefore it would make sense to make duplicates and as mentioned, store at least one off-site. If you have multiple copies at home, keep at least some of those completely disconnected from any devices/power, since power surges can also cause failures.
Something like a Time Machine drive is a good idea to have running constantly as an immediate backup in case you accidentally delete files from the computer, the computer crashes and you need to restore, etc. But I wouldn't use one for a photo archive where most of the data remains unchanged for long periods of time.
I'm personally wary of RAID devices as backups because the RAID system adds another layer of complexity. For example, if it's a hardware RAID, and years down the road the box dies, can you replace it with one from the same manufacturer that will recognize the old array? With simple HDDs, you don't have to worry about this. But, as I've also discovered, it appears at least some external hard drive solutions behave similarly. For example, I bought some Seagate Expansion USB drive enclosures because they were cheaper than buying the bare drives. My goal was to pop them out of their housings to use with my existing drive bays. But before doing so, I formatted a drive and copied over some files, as a test. Everything was fine. After removing the drive from the enclosure and placing it in my usual HDD dock, the contents of the drive, as well as the formatting of the drive, wasn't recognized by OSX, which asked me to initialize the drive. This tells me that the enclosure is not just a simple pass-through housing, but is also adding a layer to the equation. From my understanding, a fairly common problem with external drives is the enclosure going bad. You could recover the physical drive from it, but it may not necessarily be readable by the computer unless it's back in another identical enclosure. Therefore I prefer to remain with bare drives used in drive docks, allowing them to be easily used and recognized by a variety of HDD interface devices.
For cloud storage I would recommend you convert all of the RAW files to higher quality Jpegs with reasonable compression, and upload those. At least this way you will have a version of every photo in the cloud, even though they're not the RAW originals. While you're doing this you might flag your most favorite images and upload the RAW originals to the cloud as well.
For cloud storage, you should deal with an established player who will maintain a longterm presences. Something like Google Drive is probably good. Flickr might also be an option, though recovering those files will probably mean having to download each one individually. I guess Apple now offers something too. PhotoShelter is also good, though probably not the cheapest.
I use Amazon's S3 (Simple Storage Service), which is the actual storage space used by a lot of cloud storage vendors. http://aws.amazon.com/ They're likely to be around for a long time. You pay for how much you use, and it's not too bad. If you opt for the reduced redundancy option, it's 7.6¢/GB/month (though this can vary based on the geographical region of the server group you use - I see you're in Portugal). With about 150GB uploaded, I'm at around $12/month currently. And they're constantly reducing their storage pricing. When I first started a few years ago it was closer to 10¢/GB/month. S3 can be accessed through Amazon's online portal or via an ftp application that supports it. Amazon also has Glacier, which is about 1¢/GB/month, but it's a near-offline long term solution for data you don't anticipate requiring access to, even infrequently.
Hope this helps somewhat.