* - Two piece, open cell wetsuit, without a zipper. Why is that (why does it have to be two piece, open cell and without a zipper)?
It needs to be 2 piece because zippers are a bad thing. Zippers are a water vent. Good wetsuits are not wet on the inside. 'Open cell' is a misnomer, but essentially means there is bare neoprene rubber against your skin, which ideally means less or no water between you and your suit.
* What does the 2/3 and 3/4 and so on mean? Does it mean that the suit is 2mm in some places and in other places it is 3mm, or something like that?
Correct.
* Can you dive with a surf wetsuit? What's the difference?
You can, but ideally you want a proper freediving suit: form fitting, quality rubber that resists compression, and the above-noted qualities.
* What wetsuit would you recommend for a beginner? For warmer waters, about 75 degrees F.
A 3mm or 5mm suit would be fine, depending on what you want to do and your metabolism and fat percentage.
Thanks in advice!