John, I am pleased you have taken the plunge. It is a great feeling when you complete your first scratch built wagon or any other model for that matter. I first started making models over 50 years ago and building 7mm wagons from scratch over 40 years ago. Sure you will make mistakes, I still do on a regular basis.
I was lucky in 1985 I got a job which left some down time in the working day. This was not always the case, some days there was none but some days there was plenty. For the first couple of years I did what the fella's did and sit and read or play cards untill I realised that if I took a few bits of plastic and some hand tools I could build models and get paid for it as well.
I got my leg pulled before I was left in peace. The down side was my workmates would bring all there broken items to fix because this bloke makes things.
I am building a pair of Dia 13 timber wagons at the moment that are nearly finished.