Time for external HD?

You've got 13% free space on your HD, which is more than enough to avoid system sluggishness. The rule of thumb is 10% but I've pushed SSDs on my Macs to as little as 2% before I saw performance issues on both Lion & Mt. Lion. Obviously YMMV but I definitely don't think the lack of free space is the sole cause of sluggishness.

If you've got files that date back to 1988 and every previous release of OS X I suspect you've got a ton of extraneous plist and application support files. Files, that in a high enough concentrate can create indexing issues and even on a SSD cause some sluggishness issues, especially for the purposes of accessing data through Finder or Spotlight.

There's several ways to prepare your machine for migration to a new unit. None of them are anymore delicate than another, it's all a matter of how much you want to bring over with you.

A.) A simple drag & drop from the source HD to the destination HD is easy and straight forward. I'd recommend something like that for media files and/or documents.

B.) Cloning the drive using an application like SuperDuper! or Carbon Copy Cloner would also work, they've very easy to setup. Carbon Copy Cloner is no longer free, SuperDuper! is but all of the "good stuff" is locked behind a "pay for" registration.

C.) Time Machine would also work for the purposes of backing up and then migrating to a new machine. It's not awesome for allowing access to specific content within the backups but it'll work for a full and total system backup then restoration on the destination drive. This option probably wouldn't work out well if you're looking to move from two separate systems into one.

I personally have three systems at home that I actively maintain: Mac Pro, MacBook Pro and a MacBook Air. It was a total nightmare to keep all three in sync with one another and in unison with software updates and software versions. Probably the best decision I made was to move the Mac Pro to a server (Lion server) and then have the two portables as clients in my home environment. All of my data is stored on a pretty basic NAS setup and everything is identical (all data, all applications and all OS versions). It's definitely more advanced than necessary for a typical personal/consumer user but it's something to consider and can easily be done on a budget with a Mac Mini and something like a Drobo.

I'm only recommending such a solution because of the volume of data you have, multimedia data. I'm sitting at a hair over 6 TBs of multimedia and 95% of that is content that I'll probably rarely watch, it's more "have it, so I know where it is." For the day-to-day stuff that's stored locally and when I want to sync up the iPad or iPhone with media I can easily call upon the massive media library without having a percentage of that just tying up space on a much smaller SSD or traditional HD.

At a minimum I'd definitely considering you explore getting your data in one place now versus waiting to do on a migration to a new machine. Because if you can get everything in one cohesive package you can then clone that out to both of your current machines as well as your soon to be new machine.

In your music folder (~/Music) you should have your iTunes folder with all of your media that is affiliated with iTunes. That's assuming you kept the default option of keeping your iTunes folders organized and media relocated to that location. If you did not setup iTunes to operate like that then your media maybe scattered all over your HD and you'll need to track it down.

Assuming everything is in the ~/Music then you can drag & drop the entire folder onto the external HD. It's definitely possible to maintain an identical iTunes library across multiple computers, perhaps on the external HD. That way you can just plug the hard drive into the computer and tell each machine's iTunes to use that library. You'd obviously want another external HD to serve as a backup to that iTunes library.

It's difficult to identify what exactly is the reason for the sluggishness on your computer. I don't suspect it's a hardware issue but you never know. I find sometimes in troubleshooting, making a new user account can help identify issues a bit further. If the issue persists (sluggishness) in the new user then I move towards backing up all my data, wiping the machine and seeing how that improves performance.