Note, though I personally could not tell the difference in sound quality when comparing between Cubase VST 5/32 & SX3’s audio engines, others have insisted differently…maybe they were born with dog ears.
I don’t have experience with the latest Cubase versions (nor latest Windows versions) myself, so I can’t say what features are currently available. I personally don’t have a need. I’m still not clear dlbeaty on what it is you’re missing between VST 5 & LE5? You say you haven’t run out of tracks or VSTi slots on VST 5, what about LE5? I wouldn’t tell anyone to spend more money unless they had a need, though if you ask many who’ve upgraded every version ‘just because’ they’d likely tell you it’s worth it. I’ve never been a power user myself. All I need are audio tracks and midi tracks for my 3rd party VSTi’s & my external midi gear. I’m not big on using a lot of FX’s. To me, it sounds like whatever SX3 has to offer, that would be all you’ll ever need.
As for 3rd party VSTi’s running in older Cubase versions…though it seems most VSTi developers are dropping compatibility with older OS’s unfortunately, there are PLENTY that run in it fine, even with Win XP 32 bit. I’ve already got all the VSTi’s I need, and I don’t have a need for the latest versions of them to run on a Win 64 bit platform. Well, it sucks for them that I won’t be buying it. Thankfully, there are still a few developers of software & hardware that specifically say it’ll run on Win XP…that’s the way it SHOULD be imo, I’ve read a lot of users are still running on XP.
Regarding a used copy of Cubase, I wouldn’t worry about it. Anything near $200 is a great deal for SX3…for the seller that is! It should be a lot less imo. I remember seeing Cubase VST 5/32 for sale, for like $30.00 WITH the big blue dongle & original box, hardly illegitimate. If it were me, I’d offer $50.00 to no more than $100 for SX3.
Then again, ask yourself if you think it’s worth it to upgrade your LE5 to Elements 7 as recommended above, that would probably be all you’d ever need, though Elements 7 may not have any benefits for you than SX3 does, depending on your personal needs. If it’s much less money than SX3, the choice may be clear…just as long as it will run on your computer without having to upgrade that too!