New or Old Locomotives and Rolling Stock

That is one of the reasons why I collect plarail/tomy/trackmaster trains.  I did look into collecting HO train when I first start my "train" hobby.  Like you said, the rollng stocks and engines are quite expensive and not to mention it can get extremely complex when you have to install wires here and there.  I already have a headache just to install EZ Bachmann tracks.  

If you are into model trains, I would only buy used rolling stocks and not engines.  Rolling stocks are easy to fix.  Engines on the other hand requires a lot of skills like soldering and parts replacement.  The drawback of the used market is that you may have to acquire more than what you need as the market place may want to sell their items in lots as oppose to individually.    I wouldn't bother to repair trackmaster/tomy tender engines (the ones where there is a wire connected from the firebox to the tender). 

If you are still young, don't spent too much money on model trains as you may lose interests down the road.  I initially started my train hobby with wooden railway and then I move to battery operated trains.