While you can build a model railway using just off the shelf ready-to-run locomotives and rolling stock, there are plenty of kits available that allow you to expand your railway in interesting ways. As I mentioned in a previous post, depending on the time period I choose to model I might have to at least re-paint some ready-to-run models even if I don't have to resort to building kits.
My previous experience of building kits was with Airfix aeroplanes as a child. While the models usually went together alright the painting and finishing usually left a lot to be desired. Given this I decided it was probably sensibly to buy a cheap kit to see if my skills had improved over the years before possibly moving on to building expensive locomotive kits.
Parkside Dundas. This design dates from 1923 and so doesn't quite cover the entire period I might wish to model, but it's a good starting point. As you can see so far my progress is okay. It still needs some cleaning up (including levelling the buffers a little looking at the photo) and then it needs weighting (it's so light if I try and push it around the track it jumps off at every set of points), some gaps filled with putty, priming, painting, transfers applying, and finally varnishing. In other words there is an awful long way to go yet so you can safely expect a few more posts as hopefully it turns into an acceptable model.