Plug it in a to a UPS, not directly to wall power, not a weenie pathetic "surge" strip. You want to give it stable power, as unstable will cause internal caps to fail.
LOADING is a generic catch-all error message from JVC. It was really lazy and stupid on their part.
When you plug in the unit, it will post. LOADING.
There are actually variations, and the power save setting (on DVD side) can change the post messages, as it comes out of a different state.
WAIT is another.
When LOADING sticks for more than 1 full minute (literally more than 59 seconds), it means ERROR. It's not necessarily the caps, but often is.
"New" now means nothing. Caps age, storage conditions matters. If it was in a non-temp-controlled outdoor storage for the past 15 years, in somewhere like TX or AZ, then the internals are screwed, lube evaporated, belts rotted, caps dried. If it was indoors, decent RH% for years, then perhaps it is fine, though the mechanics may stick from never being used.
Understanding all this, plug it in, turn it on. See what happens.
For first use of the deck, run a full retails tape, a movie. Use a tape you don't care about, such as a ratty old copy of Home Alone or TMNT. I have fodder tapes for my refurb work, and they get chewed and everything else. I don't care, that's what those are for. Play some, stop, repeat, FF, REW, play again, then let it go. Perhaps do another stop/FF/REW in there. FF and REW while playing, too. These are all tests of the transport and deck mechanics.
-sourced decks are junk these days, and ruin the test tapes. At best, those used units sorely need attention, re-grease, realign, etc. It is tedious, and a lot of work. The new units somewhat helps here, maybe, but still may need grease. And alignment drifts due to gravity, it's not immune. You can buy a NOS deck, and it's out of alignment.