20" is fine and handy maneuver for 600y. If you think 1k and beyond is likely I'd definitely go 24" to wring all the velocity possible from the Creedmoor. Remember that all of the non-magnum .264 class can be hard to spot for as you approach 1k and beyond.
The TAC A1 is a hell of a deal but 6.5Cm available only in 24". That said, for only $600 or so more you're getting a
folding chassis, 2-stage trigger, brake and 3rd magazine. The magazines are compact for 10 rounders but spendy. Some guys don't like the cold or burn of a bare metal chassis in temp extremes. Last I looked the cheapest aftermarket folding chassis runs $700+ minimum.
Scopes. Budget usually budget narrows choices for your application. Can you provide more info, like
this from snipershide?
Mats. Padded mat with stake loops or grommets so it doesn't blow away or up on you and the rifle.
Triad checks all the boxes for me.
Bipods. Still using an old TRG I lucked into, but the B&T Atlas and others are better optioned for half the going price of a new TRG. Nothing wrong with the old Harris, too.