Hi,
I have not built one but this is a short (west side lumber) flatcar, and as such is the easiest kit to start with - you have a kit from a good manufacturer, and have made a good choice!
Always remember that the old maxim, 'measure twice and cut once' applies. The trucks could be complicated, take your time, and again try to fit everything, and before any assembly understand how the truck is assembled.
The chassis is kept secure by the deck , a solid deck could well be easier, but a individually planked one can look bettre - if the latter is chosen fix something temporarily at the other end to where you start to hold all together whilst some stiffness is gained. Flatcars were always abused, and the deck can be weathered, and 'bashed about' as much as you want, including the edges of the planks, at the outer edges.
Best of luck with your first kit; it should and I am sure will be a good looking and possibly a well used little old flatcar!
Any difficulties, add a message and I am sure that MLS will assist, photos would help with describing any troubles that you have, and certainly one or two of the finished article will be appreciated.