There's nothing to say that it would be any harder for Spike to regain his mortality than it would be to regain his soul. Possibly even easier, since it's fairly common knowledge in the demon community these days that a few drops of Mora demon blood will do the trick. Barring that, one of Buffy's best friends is possibly the most powerful human magic practitioners in the world. If there's a re-humanizing spell, she should probably be able to find it.They cannot get old together, or have any children. Buffy tried to abort whe she though she was pregnant because she knows that her child will always be in danger.
Once Spike is human, there's nothing whatsoever keeping him and Buffy from moving away, finding some peaceful little town, and starting new, normal, and probably quite boring lives together, away from vampires, demons, and the annual threat of apocalypse. Cute little cottage, white picket fence, rocking chairs on the front porch, the works.
These are characters living in a fantasy universe where literally anything is possible, with the single possible exception of getting Angel to laugh.
That's the "could" part. What tosses the cat in the buttermilk isn't a question of "could," as much as "would."
Buffy would no sooner walk away from being the Slayer than any other superhero would walk away from doing what they do. There's always going to be that old saw about, "Someday, when my job is done, when the world no longer needs me..." We've already heard Buffy's version in the cookie dough speech. Those cookies are never gonna get baked. Buffy knows it, Spike knows it, Angel knows it, the whole Buffyverse knows it. But that doesn't mean that it isn't fun to dream about once in awhile, and that's all that Buffy is doing, just dreaming about it.