Head is actually not related to lifting height. A three story home and a single story home will have the pump seeing the exact same head if the piping lengths, diameters, number of elbows, etc... are otherwise identical. Head is therefore a product of only these things. I.E., pipe diameters, lengths, elbows, tees, valves, etc. Height does not enter into it.
I believe that to move 250,000 BTU's of output per hour requires 25 gpm of flow, and I believe that calls for 1.5" I.D. black iron pipe. For PEX I don't know what would be called for.
BTU's = GPM x 500 x delta_T (where delta_T is generally assumed to be 20 degrees of overall temperature drop across your baseboards or through your radiant floors)
Therefore: BTU's = GPM x 500 x 20
Rearranging for GPM you get:
GPM = BTU's / 500 / 20
Then solving for 250,000 BTU's gives us:
GPM = 250,000 / 500 / 20
GPM = 25
25 GPM of flow will deliver 250,000 BTU's at the standard 20 degree temperature drop, meaning that if the water departs your boiler at 180 degrees and at 25 GPM of flow, and then it returns to the boiler at 160 degrees, 250,000 BTU's per hour are being delivered somewhere.
Since you don't know the actual head of your system, you can't look on a pump curve chart and know how many GPM's your circulator is actually delivering, and therefore you can't calculate your pipings maximum potential to deliver BTU's.
I don't believe a Taco 009 can achieve 25 GPM at any head. A Taco 0012 seems like it has this potential at up to about 10 ft. of head.