My advice would be opposite that of DKW, go with anything 9th+. The newer IIDXs have more options and IMO better music, not to mention the newer high-res interface you're used to. I have the feeling you'd be disappointed if you're first import was an old low-res style, especially anything pre-6th. Plus, those are harder to find.
So for your first few games pick up the newer releases, and pick up all the older games later if you want. You'll be able to appreciate the older stuff once you're a true IIDX addict, whereas if they were one of your first games you could end up feeling kind of shorted especially since you started on BMUS.
I would personally pick up Happy Sky because it rules, but so do RED and 10th, and (to a slightly lesser degree) 9th, and 6th-8th are also good but I'd get a 9th-12th first. 3rd-5th are pretty old (and it shows) but still worth picking up eventually. It's up to you, listen to the music and make your decision based on that. I wouldn't worry about difficulty in new IIDXs either, since you're not a total newbie and there's always beginners mode. Once you can do ~4-5s or so any style is good to play on L7.
You know what I'll make it simple. Get Happy Sky, especially if you can only get one game for a while. | 
