You can get a decent brand bike at any bike shop in the $300-$400 range. All will be good with sizing and fit. At this stage all of the mod range bikes are kitted out more or less the same. All with give you a good bike but most come with a basically useless suspension fork.
If you wanted a little more money Frog bikes are awesome in my opinion. Better components than the Trek, MEC, ghost, speialozed, giant kids bike and significantly lighter weight. I could never justify the additional cost but there is definate value in them.
https://www.frogbikes.ca/lightweight...es/frog55.aspx
But really used is probably your best bet. Go to a bike shop to get a feel for sizing and fit. Also talk to them about servicing. Then shop kijiji for something that works and be picky about fit. Then if you end up finding a good deal and don't want to do the bike tune up yourself take it to the shop where you got fitted to do the work.