$300 for an FT-817ND sounds about $180 less than I would expect to have to pay for a good working unit in a reasonable condition, so I suggest you take a good look at it before buying. If it’s in good order - grab it, it’s a bargain.
The one thing that the X5105 has that they FT817/818 range don’t have is an internal ATU. The X5105 is a current design, SDR based rig, so the receiver is probably somewhat better than the old design FT817/818. But is the new X5105 worth another $300 dollars over the used FT817ND, probably not.
PS: If you do go for the FT817ND - two modifications you might wish to consider - The 2500 mAh LIPO battery upgrade modification and RF-Clipper type speech compression. OK you can add an external ATU as well, but as long as you use resonant antennas, you wont need an ATU.
PPS: Take a look at the XIEGU G90 as well - I’m not sure what the price is as it’s very new but it’s basically the 20w version of the X5105 and replaces the X108G. Going from 5 watts to 20 watts is noticable, where 5w to 10w isn’t.