Thanks, I didn't wanna step on toes =)
Couple questions: Is the QB program a server based program or are you just wanting to store the files on a server? Is there really a need to transfer files between the computers? What I'm getting at, besides the files that the QB program saves and you use, why do you need the computers networked together?
Because I think you can solve a few things with one service: cloud storage. My only experience is with Google Drive tho, because it does what I need it to so I haven't looked anywhere else. But you just install Drive on the computers, save the QB files in the Drive folder and they are synced to the ones online automatically. So they are available anywhere, you can set who has access to them and they're backed up. And 5 GB is free. To get more it's $4.99/month for 100GB. That is a great price.
If you want the computers networked together I'd go wired. Running the cable with conduit is fairly straight forward, not that hard so you could keep costs down and do it yourself, and would be the most reliable. I'd do Cat6 and get gigabit all the way around.
Wireless will do but I'd only use it as a last resort. The distance outside isn't that big of a deal, you can get directional antenna if needed. I do love wireless but my issue is I love it when it works and want to kick a kitten when its not. It drives me insane, "It's RIGHT there you stupid laptop. CONNECT!!!" I have had that convo a number of times.