HI,
Welcome, I have to agree with a couple of the prevoius responses. Bigger is a bit better for training. However if you already have a 40 size engine that in my opinion outweighs the benifits of going bigger. 40 size trainers are fine. I also agree with going with a basic inexpensive ARF.
Trainers WILL get bonked around. They WILL need repairs so basic is easier to fix. The K&B should work fine. You might consider new o rings especially on the needle valve. You also might
do a search on exaclty that motor for things to watch out for and tricks to make it run well. Some of the K&Bs had marginal carbs. After market replacements were common, they even came with Perry carbs on certain models. Perry carbs are still available for them by the way.
Good luck its a fine hobby! oh yeah remeber all arfs need a good going through. Fire wall, landing gear blocks, wing hold down blocks (if bolt on), check hinges WELL, check glue joints such as rthe tail feathers WELL. Bigger wheels make like easier on turf.