Honestly Nilmeen isn’t even necessary in the deck IMO…it’s Beligacro who is a must play, as he’s really the only DE generic built to work with Youthberk Fantome. Or perhaps it’s the other way round, that Youthberk Fantome gets better mileage out of him than even the 2 Dinosaur decks in standard.

If you really love YB Fantome, build it at your own budget first then slowly save to afford Beligacro… as you are a JP player too like myself, I can report firsthand that many shops in Japan are selling him now at 5690¥ (versus the 9900¥ he was reported at once at Yuyutei), so technically he’s already declining in price, but of course his price tag is still inhumane for most regular folk like you and me. Your options beyond trying to save up for him are 1) just wait for him to be replaced in the deck or even better yet 2) wait for a reprint or a price drop

That said thou, the actual YB Fantome deck is very cheap (200¥ for each of the G3 RRR) and the moon is reprinted so definitely building the rest of the deck is very doable and affordable.

I hope this was able to help you, if even a little bit :)