Migrating a MBP contents to a newer OSX

Hi, I am going to sell an MB Pro mid-2010 which has OS 10.8.5 on it. I want to

A) save all my data/files to some backup that could be accessible from ANY newer

Mac. B) ideally transfer all the contents to one of a couple newer MB Pros which have

SIerra on them.

What's the most simple and straightforward/bulletproof way to approach this? I realize

some older apps may appear as incompatible in SIerra or El Capitan,whatever..but that

isn't really a big deal. Mainly just looking to preserve all data and as many apps as possible,

and ideally carry over whatever prefs can be preserved for respective apps et.al.

Can I just use migration assistant for this? presumably this involves a target FW mode.

Will there be permissions issues and if so, is there some default way to make all contents

accessible? Sometimes in the past when I've done this some folders/files will have 'can't

access' protections on them where certain things can't be read/written-to ..

Thanks!