you can buy a frame for pretty cheap. but even getting good ebay deals piecing one together you'll still be cheaper off to buy a blown up 250F or 450F and do an engine swap.
this way is a LOT less work and parts sourcing.
i know the triple trees and a few parts are different on either '12+ or '13+. so saying that you may have more parts availability going with a pre '12. thats going to need some researching to see what year they made big parts changes. if that even matters to you at all.
on engines, dont quote me but i think '98+ are all the same, possibly later than that. so no need to hone in on a particular year with a model that late.
a 450F frame will have a bit more room, but 450F's dont grenade *AS MUCH* as 250F so will be slightly more expensive or harder to find.