If you want a cataraft for 2-4 people, 14 foot is likely OK, you may also consider 15 and even 16 ft. For rafts, length would be 1-2 shorter to get similar load and performance.
1. Hypalon is synthetic rubber. There are different kinds, from very light black hypalon ( used in Shredders) to very thick used mostly for commercial boats.
Hypalon is stronger and more durable than PVC, it has a lot better UV resistance - yet if used with same thickness, hypalon tends to be about 15-20% heavier. It is also more expensive and is a lot harder to repair than PVC. Hypalon boats can be rolled and stored deflated - though it is less than optimal way of storing the boat.
2. PVC is a most common inexpensive <raft, cataraft> material, with decent durability and weight/strength ration. Most PVC materials degrade in 10-15 years if they get a lot of UV exposure. Some PVC materials are better than other (e.g Aire materials).
It is very advisable to store PVC boat half inflated, especially in temperature in storage can drop below freezing. Frozen PVC can get permanent cracks ... if your boat got frozen then rolled, do not move it or unroll it till it is thawed completely.
3. PU coated / impregnated materials. Generally, about twice as light as PVC and still stronger than PVC. Great wear and tear-resistance while wet, but may be easily damaged when dragged over dry rocks. Significantly more expensive than PVC, and even than Hypalon. UV resistance greatly depends on coating - you need to ask manufacturer about that.
Common sense says you only need PU materials if you want ultra-light boat.
OLD or NEW? Depends on price and condition. Probably, best if you can find 1-2 y.o.
boat still under warranty - but at a significant discount. Some new companies
like Saturn or RMR sell decent boats manufactured in China for low price - and their quality is good enough to consider them along with used boats by US manufacturers.
NRS tubes you consider - they can be anything... from bargain to garbage: depends on CONDITION, also - on a year, model, materials used.
You can go with any manufacturer, if you get a cat in great condition: Sotar, Aire, NRS, JPW, STAR, etc.
Here is one more option to you: look at our company's site zelgear
Check the Otter-4 frameless (inflatable frame) cataraft. Judging by what you said about the storage, it may be a great option for you. I have one demo R4 that is less than 1 year old; I can sell it for $1500 ( about 60% of a full price). While it is a cat, with it you do not need a metal frame if you paddle ( but can buy and install a standard frame if you prefer to oar). The advantage is low weight and easy storage ( without frame), also kneeling seat give you much better paddling position.