The USBPre2 is a very nice piece of gear; I owned and used it for several years. However, its main benefit is that it can be USB powered and that it connects digitally to recorders, which it cannot do with an M10. So that plus an M10 is not a good match; if anything, that plus a used D50 is the smarter move.
It is true that recorders lose their value more than other equipment; that's why I ultimately went with a Zoom F8 and not a $5500 Sonosax SX-R4+. But I wouldn't just take any piece of junk at the most cut-rate price and go with that. Most of the cut-rate junk, like virtually every offering by Tascam, costs that little for a reason. If you look at the threads, you'll see lots of "little quirks" and problems that add up to a big pain. Again, I'm not just making that up; I've owed at least three Tascam units.
Personally, I also prefer an all-in-one solution, rather than multiple different failure points in the chain. Decks like the Roland R-26 ($500) are excellent as all-in-one boxes, plus that deck also has two extra channels you can use for an SBD or with an outboard pre. The F8 is, so far, a very nice piece of equipment, robust in build, with lots of options, and it can be controlled remotely by an app on an iPhone, which to me is a huge benefit. However, it might also be more than you "need." I'd also look for a used Edirol R-44, which can be had for about $550. That thing is a rock-solid piece of equipment that is easy to operate, sounds good, and works correctly every time.
I guess if you go to only shows with a lot of other tapers, it doesn't really matter if your gear fails from time to time. With most of mine, if I screw up, there won't be a tape of it, so I prefer reliability and certainty of execution over all things.
Also, further back it seemed you were considering Schoeps. If you are (and you don't necessarily need to; the AKGs are great mics) I would not buy some AKGs as an "interim step." You'll just end up basically making the Schoeps you actually want more expensive by buying an incremental upgrade that you're going to change out later.