RE: common arf mistakes?
Good advice above but I do have an exception. Don't thin epoxy unless you need to. If you'll purchase some "Finishing Epoxy" and use it only to fuel proof firewalls and tank compartments you won't need to thin it. Regular 30 min. epoxy for joining plywood and where strength is needed.
Don't believe the times printed on a box or in an ad regarding how long it will take to assemble and be flying. Just take it a step at a time and just keep going until its done. The box may say 6 hours and it may take you 6 weeks - just don't rush.
There is more than one way to do things so if something doesn't seem right - STOP and think about what it is you are doing.
Use good tools and only buy one when you need it and get a good one - you'll probably need it again. This is more true with things like needle nose pliers, ball drivers, screw drivers, etc. Power tools don't need to be heavy or powerful. A good hand razor saw/miter box combo is worth it.
Spend your time getting the plane set up like you like it at the start. Get the engine you want and the radio you want. Its far better to spend the time up front than to constantly be redoing it later.
Search this site for a "build thread" on the model you select. The tips you'll pickup are invaluable not to mention some online friends.
If you don't find a "build thread"; start one. Again, the tips you'll pickup are invaluable...
I believe a .60 size trainer is easier to see and a .90 four-cycle nicer to listen to.
Plan on having fun,
and Happy New Year,
Clay