Hey Karen,
The most important thing about paddles is that there right for you and that you try as many as you can to find the specifications you want. There are plenty on factors that can personalise a paddle to the paddler and fine tuning your paddle specifications can take years to get right. However there a basic ones take you can play around with when buying your first.
The first thing you need to know is wether your a play boater, river runner or you don't know yet!
Blade type:There are two basic blade shapes. the first blade is designed for instant power from the stroke and for paddle strokes around the bow. this is show in the following image.
These blades are commonly used in play boating, however river runners tend to like them if there style involves plenty of short snappy strokes.
The second design, show below, is more suited to river running/creeking.
This design of blade offers more even power from the start of the stroke to when blade is pulled from the water.
Blade design is the most important one to play around with and try other peoples paddles.
Blade Size Too big a blade will be difficult to pull through the water and may cause muscle injury. Too small a blade will mean too weak a stroke to move your boat to where you want to go.
Baiscally ask your self 'how strong am I?' then choose between small medium or large.
Shaft diameterPeople with small hands can get paddle shafts with a small diameter. This allows a fuller grip around the shaft.
Try this out. you don't even need to take the paddles out on the water. just see how various diamters feel in your hand when your in the shop.
Shaft typeStraight shaft or crank. Straight shaft paddles are cheaper. Cranks are more expensive but offer more comfort. people debate if the cost is worth it but i think there great.
This one is really just a cost based choice.
Paddle lengthThe lenght range on paddles is from 185-205cm, however the typical length is 190-197cm. Short paddles tend to be for playboaters as can be side ways, upside down, spinning etc and paddles can get in the way. Long paddles tend to be for river running/creeking. Longer paddles offer more stability and power.
Smaller/petite should try lower down on the scale, while bigger stronger people should be higher on the scale.
For example, if your small and into playboating try 190cm, if your bigger and into river running try 197cm, or 193/4cm can be middle of the road.
Its important to try this one on the river and remember a couple of cm can make a huge difference.
FeatherThis is the angle between the blades. Club blades are always 90deg, however this feather is only useful in very windy conditions. For whitewater paddlers 45deg to 5deg is typical, however up near 45 deg is more popular. 45 deg offers more comfort than a club blade as there is less rotation of your right hand.
5 deg paddles are very specialised and offer no rotation of your right hand . However it is very difficult to paddle with any other angle once your used to this tight angle. To be honest i paddle with zero feather before and its not all its craic'd up to be.
SafetyNot a big consideration of beginners/intermediates however for advanced paddler on grade 5 rivers its is very important to have a strong paddle.
the grade 5 envirnment can be harsh on a paddle and i have seen some horrible grade 5 swims because of broken paddles.
Rough stuff have given themselves a bad name in recent years for reliability of there paddles, however brands like werner and AT are bomb proof.
Overall Costexpensive blades means that if you lose you paddle its a right kickin the stones.
Second hand bladesI wouldn't recommend second blades for grade 4 rivers as you don't know where they've been.
Also as paddles can be personalised, it may be hard to fined the exact paddle you want.
However, if you find the paddle you want and your happy with the condition go for it.
Hope this helps, theres alot in there and to be honest there's not much out there for 100 yoyo. i recken for 150 you could find what you want and get it brand new.
Tom