[color:"orange"] 1. Do you need level 4 extract, or combine?[/color]
You need level 4 extract
[color:"orange"]2. Do you need 1 of those temporary stat boosting potions?
3. If you do, does the size matter? [/color]
I don't even know what you mean by this.
So, the answer is no.
[color:"orange"] 4. Do you need alchemy plants? [/color]
Yes, you do.
[color:"orange"] 5. If you do, does anyone have a formula list for the different potions? [/color]
Please look
here [color:"orange"] 6. How many stat points does it give? [/color]
I believe it was 6 stat points per potion
[color:"orange"] 7. Is it worth respawning battlefield merchants to just try and buy tons of Holy Water? [/color]
That really depends on you and your playing style. I found you can get through the entire game (even on hardcore) easily enough without needing any permanent potions...