1a) I downloaded a mod (http://tesnexus.com/downloads/file.php?id=23069)that let's me delete spells from my spellbook. This question is for anyone that has used such a mod. I notice that when I go to create a spell, the list of possible effects are based on spells that I've learned. If I delete, say, all the fire spells from my spell book, would I lose the ability to create fire spells?

1b) My intention is basically to have only custom spells. If I delete all the vanilla fire spells, but still have a custom fire spell, will I still have access to that effect for spell creation?

2) I was goofing off with the spell creation tool and tried to see what it would cost to create a vanilla spell I already had (Weak Fireball). I noticed that for the same effect, my spell would have a much higher magicka cost. I'm assuming that this is because my skill level was rather low. Is there a point where I can make spells with a lower casting cost (or at least as efficient) as a vanilla spell of the same effect?

Thanks.

As long as you have any spell with the effect, you can always use it for new spells.

I believe that there are some spells you can't delete. The default healing spell would be an example, since it's a "placeholder" for your spell slot (it always gets selected after you cast a scroll spell, for instance.)

On your second question, I believe that Weak Fireball is one of several spells in the game that are more magicka efficient than you can custom make. Wizard's Fury is another.

1a) You'd only lose the ability to make spells with a certain spell effect if you delete all with that spell effect, of the ones your character is able to cast.
1b) Yes, again, provided your character has the skill to cast it. Although, as Glargg mentioned it is HIGHLY recommended you keep the default Heal Minor Wounds spell. Your game may crash in certain situations if you do delete that spell. AFAIK, it is okay to delete any other Oblivion default spells.

Tip regarding Spell Delete and Item Remove: You can go and buy the vanilla spells again that you can normally buy, if you had already bought them and deleted them out of your spell book.
Also, for future mod related questions, usually you get better answers for them if you post them in the [url=http://www.gamesas.com/index.php?/forum/25-oblivion-mods/Oblivion Mods forums.[/url]