*SO, I could purchase a 540TM WTE new from Amazon or a used 930T from ebay (traffic service already expired, maps updated this week). Not getting the lifetime map updates with the 930 is nearly a deal breaker.
First, understand that the lifetime map updates on the 540WTE are only for the continent where the unit was sold. If you buy in the U.S., you'll get lifetime updates for the North American maps, but not Europe. If you buy in Europe, you'll get the lifetime updates for Europe, not North America. Annoying, but a fact of life (that none of us appreciate or understand). Further, there isn't a lifetime map option offered for the 930. The only units where it's offered retroactively are the ones that now offer it as an option (they're all more 'recent' models).
*Any drawbacks to getting a used unit?
Only the drawbacks of buying a used anything from someone you don't know. By now, the battery on the 930 may be getting a little tired, but that's in part a function of how the previous owner used it. If you could find a FACTORY refurb 930 it might not be a bad deal, and you may well be glad you've got the extra memory slot on that one. More on that in a moment...
*Is the 540 a solid unit and not to worry about the lack of more memory available?
There's an issue with the European map size on the 540WTE that may or may not bother you given your own plans. A map release earlier this year demonstrated that the full Western Europe map wasn't going to fit in 2GB of memory any more. So you wind up loading some portion of the W.E. map at a time from your PC to your TomTom. If your plans in Europe keep you in one area most of the time, you can keep that area loaded with no problem. In fact, while in Europe, you could just offload the entire U.S. map to your PC and keep all of W.E. on your unit.
Here's the thing -- if you plan to spend 3 years in Germany, having the "lifetime map update" tied to North America really isn't going to be of any value to you. I'd think you'd rather have updates for Germany and other parts of Europe for 3 years. Based upon that, I would strongly recommend that if you plan to buy the 540WTE, you do so in Europe where you'll get the free updates for Europe (remember, you only get one or the other), and deal with the North American situation when you get home. Of course, beware the prices in Europe and the exchange rate and all that. Who knows, it might be cheaper to buy the WTE here and add the cost of the lifetime European updates. Just do that shopping/calculation carefully.