Barbaros, a Mature Nephilim, 2 Young Nephilim and 2 upgrades comes to 25 Soulstones. It looks cohesive and will only cost you a Nephilim box and it doesnt overlap with any master boxes. Its 5 stones over the Henchmen Hardcore format and I dont know if its competitive, but that shouldnt be an issue for demo games.
A 20 stones crew could be Barbaros, Mr. Graves and 2 Terror Tots, and 1 upgrade. Its cheaper than the Nephilim box, sticks to the format but it makes buying the Lilith box later an even worse deal than it already is.