The thing is even if a Buk system is the system confirmed as the one which was used,the question moves over onto who used it??
We will say the separatists and the separatists will say the Ukranian military.
The thing is both sides had access to the same systems.
Some napkin maths here but... Donetsk Oblast was 10,238 sq miles in size, the Donetsk People's Republic is ~1/4-1/5 that size, so ~2000 sq miles, given the Buks detection and acquisition range (plus the missiles range) if it was the missile used it must have been fired from within the Donetsk People's Republic no?
The plane came down intact but out of control so it can't have travelled that far from the point of impact.