Keep in mind that in fiberglass construction, heavier is not necessarily stronger. Buy a boat you can afford, but if you find a boat you really like which is too expensive, it may be worthwhile to save your pennies until you can afford it. If you have a fixed price range you are interested in, it may be a good idea to only try boats in that price range, so you are satisfied with what you get. Sometimes you can find boats sold used for less, especially if a shop or outfitter is selling old boats from their rental fleet. The best way to choose a boat, and the only way to determine its paddling characteristics, is to try it, and you should take opportunities to try as many boats as you can to decide what you like.
Many shops have demo days, and symposia are good opportunities to try boats.
Try to find an opportunity to paddle in the conditions you are planning on using the boat in. Also, consider how the boat handles when it is loaded as well as unloaded. Lean the boat onto it's side. Paddle the boat into the wind, across the wind, with the wind behind you. Speed - a function of length, width, and hull shape.