If its only for a short adventure i'd just stick to the level adjustments in MM. I agree with others that Savage Species was a good idea but badly done.
It'd be worth looking at some of the Savage Progressions articles on the wizards site so you can see how they break high level monsters down into monster levels.
A PC beholder might be interesting but it might be problematic, perhaps if its lost a few of its eyes (especaially the anti-magic one) it might be a bit more balanced, or a Gauth might be a good substitute.
Alterativly let them play whatever and stick however many class levels you feel appropriate to balance them all out, if its just for one 'hog wild' adventure let rip and damn the consequences.
It'd be worth looking at some of the Savage Progressions articles on the wizards site so you can see how they break high level monsters down into monster levels.
A PC beholder might be interesting but it might be problematic, perhaps if its lost a few of its eyes (especaially the anti-magic one) it might be a bit more balanced, or a Gauth might be a good substitute.
Alterativly let them play whatever and stick however many class levels you feel appropriate to balance them all out, if its just for one 'hog wild' adventure let rip and damn the consequences.


