I would say for the most part that it depends on what level your PCs currently are and how long you want this mysterious gentleman around. I would agree that a lich or a dragon COULD be too powerful for them to deal with directly, but that assumes that both the NPC and the PCs are stupid enough to get into a straight-up battle.
One idea I've had and am hoping to get to implement in one of my upcoming campaigns is a 2e mist dragon taken in by Ravenloft's Mists. If you have access to the 2e
Monstrous Manual, you might check it out. The mist dragon isn't nearly so powerful as most of the other dragon types, and--I think--its very nature leads to some amazing possibilities for it being corrupted by the Mists of Ravenloft.
Anyway, whether a dragon or a lich, the PCs don't have to find this out. If he's a benefactor of sorts (being a good host, helping them get in touch with priests for healing, providing revitalizing food and drink, etc.) it will go a long way to earn their trust. Afterward, maybe he drops out of the adventure for awhile (assuming he's "working" other groups of adventurers the same way).
Then, a bit farther down the road, he calls on the PCs and asks for a "favor" that involves them tracking down something that was "stolen" from him--the spellbook or magical item or whatever. And maybe a bit farther down the road, he provides them with some additional aid, and has another "small favor" to ask in return.
It depends on your goals for the campaign, but there needn't be any full-on, spell-hurling, clawing/biting/death breath confrontations between him and the PCs until they (and you) are ready for it. Dragons, liches, and other long-lived adversaries are accustomed to remaining in the background, lying low, and manipulating others to destroy their rivals.
If you're into the Forgotten Realms at all (and I know it's almost sacrilege to mention FR here

), you might check out one of the new "Realms of the Dragon" anthologies. Can't remember for sure which volume it's in, but there's a story by R.A. Salvatore that MIGHT give you some inspiration in this regard.
Good luck with your campaign!!

“I let out a battle cry. Sure, a lot of people might have mistaken it for a sudden yelp of unmanly fear, but trust me. It was a battle cry.”
― Harry Dresden