From what I hear finecast isn't that bad for the most part and that the defects that you hear about are by far the minority. Also, as an owner of the metal tyrant, I would recommend getting the finecast one. The metal one is a real pain to build and it can come apart very easily if you don't pin everything. My personal belief is that the extra $10 for the finecast one is most likely worth it.
To be honest I'm not sure how useful biovores will be. To me it seemed like the biggest problems with your list were difficulties in taking out vehicles and the lack of synapse. The tyrant helps with synapse (remember to convert a pair of tyrant guard while you're at it) while removing the fexes decreases anti-tank overall. The biovores would help against lists like ork mob lists, but if you face mech or MC heavy lists then they won't be doing very much at all.
You must construct addition pylons.
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W40K: Tyranids, Space Wolves, Sisters of Battle
WarmaHordes: Cryx, Legion of Everblight, Khador
Infinity: Acontecimento Sectorial (PanO)
Malifaux: Outcasts, Neverborn