It's doable to convert a race on the fly for an NPC (CRB to ARG).
It's not as easy to do classes: turn a 15th level cleric into an 15th level inquisitor or something.
I find the higher (5+) levels more useful. I can pretty much knock up a 3rd level rogue in my head. Items are what kill me.
I would find an optional BBEG section useful though, stating what treasure it would have if it used Paizo's AP trick of giving the boss monster PC level treasure (and changes to stats if so)
BBEG: Upgrade +1 armour to +2 armour (+1 AC), add Headband of Intellect +2 (+2 int, +1 AC, +1 2nd level spell/day).
That might take up too much room (especially on higher level characters) but would let you use the PC as a Boss, or at +1 CR. Cobine with dvanced for +1 CR and one NPC per 3 levels is quite doable.