I'm familiar with them a little. They have a snotload of used parts listings on eBay. I do know that they (QualitySprinter.com, Quality Parts USA, and D & P Industries, Inc.) are strictly a parts reseller, not a rebuilder or a repair shop. They pull a lot of stuff from wrecks and have a few things rebuilt for them.
An engine from them includes just the engine. It includes the basic engine block, oil pan, cylinder head and internal components. All electrical and bolt-on parts are not included. Nor are the injectors, and in order for them to honor the warranty, you must supply new injectors, and those will run you an additional $2500, give or take. If you're lucky you can reuse the current fuel injector pump, if not that'll run you another $1700 or so.
A new one from the dealer includes anything and everything attached to the engine, including starter, alternator, electrical connections, fuel rail, all pumps (except the fuel pump, I think), tensioner, everything. It's plug and play, a complete deal ready to drop in and drive away.
If you go with a used or rebuilt engine, depending on what kind of shape your current ancillary components are, and the expertise of whoever does the swap, it might end up being cheaper to go that route, but it certainly won't be as ASAP as a dealer dropping a new one in, and it could very well end up costing more due to increased labor and parts replacement costs.
One thing I find interesting about QualitySprinter.com, being that they are a parts reseller only, is they have a couple of PDF files where they have their own recommended maintenance schedule, which includes laughable maintenance intervals of several things (like transmission and rear end oil to be changed every 30K miles).
Also, they list no physical address anywhere on the Web site or their eBay ads, which is another red flag. In addition, used engines come with a 90 day warranty and rebuilts come with a 12 month warranty, but also comes with a bunch of strings attached to that one year warranty. I'd read that carefully before ordering.
They also have a nifty PDF where they give you tips and information, and step-by-steps, on changing the oil. In it, they tell you that 229.5 engine oil "requires a special oil filter that is not readily available in the US so avoid if possible", so they are specifically recommending against 229.5 oil, because the "special" filter (which has never existed and has never been required) isn't available here in the US, which tells me straight up that they don't know what they are talking about, because I know exactly where they got that bit of misinformation. Based on that alone, I'm not sure I'd order so much as a used fuel filler cap from them.
They do have a lot of parts pulled from wrecks, both on their Web site and on eBay, and that might be a good way to replace a part, but even the used parts they have tend to be way overpriced, IMHO. I got a used EGR valve off eBay that was pulled from a 6000 mile wreck and paid $45 for it. They had one from a 10,000 mile wreck that they wanted $199 for it, but they'll sell you a new one for $250. Though, I'd want to make absolutely sure that it's not a Chinese part, which is common out there, and when you see a part that is substantially below that of other discounted online guaranteed Benz or Mopar OEM parts, it's probably a Chinese part. Another dead giveaway is when they list the part number (in the rare cases where they do), and it doesn't match a MB or Chrysler part number.
In any case, if it were me, I'd deal with a sure thing in a plug-n-play complete replacement, rather than something with too many potential problems that could end up costing considerably more than the dealer. In fact, mine's got almost 300,000 miles on it, so if something like a fuel rail goes bad, which is more than five grand, I'll replace the fuel rail along with the engine and everything attached to it for not much more than twice that. Good luck with whatever you decide.
Hopefully others (OVM, Eddie, RLENT, hint, hint) will chime in with their thoughts to give you some more to think about.