If one has enough credits, can one purchase a custom ability/spell within reason?

Never heard of it being done. I find it hard to beleive that someone wouldn't have already tried it and offered plenty of credits to do so. So I'd guess no. Even reasonable abilities might combo really, really good with certain guildskills.

I don't think there is any chance to do that, unless you're talking about really weak damage spells or simple defensive spells like a circle only, like a bearhug or so.

Any kind of affliction spell, unless it's an affliction you already are able to deal out with the exact same requirements and all, would become a big problem with balancing I'd say.

Any time the admin would add an ability you can learn, they'd have to take in mind whether that'd not give you just the extra edge that provides you with some overpowered combo or patch that one weakness they've left in your otherwise really powerful skills that usually balances things out.

And that not only for you but for anyone who bought such a custom spell, which would quite likely be a very popular purchase if it is offered.

That said, custom spells would be cool, even if they don't do anything impressive

Though, on second thought... I think Dresden in Achaea had something like that, some custom message in one of her abilities I think...

I am talking about raising the equilibrium lvl, as well as balance, and giving the ability to fly, or ....

Add a race ability? Some of them, yes. There are artifacts to do it, if not in Lusternia, then in the other games. Rings to make you fly, necklaces to make your eq faster, belts to increase stats...

If it's not already available like that, then no.