Assembly is straight forward , you will need micrometers to check all of the sizes & clearances if you are not taking it to a machine shop . You also need a good torque wrench .
IMO you need to have things checked by a machine shop , if the block is used the main & aligment & bores need to be corrected as well as having the block cleaned , there is no way to bore a block a t home without $40k in machines !
I guess it depend how complete the block & heads are but I would say yes it will cost $15 k more , the rockers are $160 each x 16 + the 4 shafts , pedestals , valves , springs & retainers ..... I figured about 8K to build the heads in parts alone
The block will need crank , rods , pistons , rings , brgs , timing chain , cam & lifters & oil pump can easily be another 4K + . I would consider a stroker if you are buying all new anyway , it is easy to hit 500 + ci .
Now you need all the tin , valve covers & spark plug tubes , timing cover , oil pan & pickup , water pump & pullies .
You still need intake & carb / carbs / EFI + dist & wires .