I can't help you much with your first question, as large molds are difficult to make and keep it cheap...
What is the general shape of the item? better yet a reference photo?
As far as the 'ice' resin and clear resin, they are at the base atleast, the same thing. there may be one or two chemical differences, but not enough for it to make a drastic difference. The Ice resin is more of a brand name resin from what I can see.
Here's an info link for the Resin types to look into:http://www.resinobsession.com/Resin-FAQ ... resin.html
You should also note that some of these can be combined to make virtually indestructible pieces.
A silicone mold can survive multiple castings as long as you use a release spray. when working with resin, always use a release, it keeps the resin from sticking to your mold and it helps to prevent impurities on the surface of your cast.
A few months is plenty of time to work on this project, the hardest part will be finding or creating a suitable mold.
After that all you will need to do is spray release on your mold, mix your resin, pour it, let it cure and take it out.
if there are any bubbles, scratches or places that haven't fully cured on you're cast after you remove it from the mold, get some sheets of low bit sand paper and sand out the impurities.
on certain resins this will cause a white film, applying a coat of resin over the sanded area with a disposable brush and allowing it to cure should lessen this film, make sure any of the dust from the plastic is washed away with a small amount of water and that the cast is completely dry when you go to apply the top coat of resin to it.
if the item is supposed to be made of ice, this reduced white film may would in its favor by giving it a slight frosted look.
You may need to order a mold offline, The link I posted is to a site that also sells many different Resins, coloring powders and liquids, molds, and other related things.
They may have some large molds, if not I would suggest searching online.
Just remember that it doesn't have to be a mold specifically made for resin, almost any flexible plastic will work.
Never underestimate things you have lying around like shaped plastic packaging (like you're christmas ornaments may sit in), glad ware bowls, cupcake molds, plastic cake molds and the like.
Almost anything will work as a mold as long as it has some thickness and flexibility to it so you can remove it from the mold once its dried.
I hope that was of some help