I would rent these as well. I don't think that you should buy anything in OW. There's so many choices out there and if you pick too early, you'll most likely regret a purchase.
Fins - so many choices, just take a look at the discussions on the board to see the variety of opinions on what to buy and what to avoid. Your best best is to try a bunch out and then buy the one you like. This can be an expensive purchase (over $100 for some fins), so it's best try a variety before making a purchase and then a re-purchase later on.
Boots - a lot of choices here as well. One thing to consider is that you should try your boot and fin together as a combo. Some boots have thicker soles/more padding and may not fit certain fins as well (or they can fit better). The important thing is to try them together, cause you don't want to end up getting boots and fins that don't work well together. If you can't try them together, you can post to scubaboard and perhaps someone will have the set-up you're interested in and is in your size.
Masks - this is probably the one thing that I can see buying for OW. A properly fitted mask will make your diving significantly more enjoyable. Not all masks are made the same so you must try them on before you buy.
Snorkel - this is only used in OW training. Some boats trips may require you to have them, but for the most part these are never used ever again after OW. Unless you're snorkelling too. . .
Where are you at and what's the water temperature? But again, I would rent this too.
But for inexpensive wetsuits that are well made, check out
Triple-L. I dive their 5 mil and am really happy. I also have a 3 mil Henderson Hyperstretch, and while the Henderson is a nicer suit, it's also 6 times the price!