Two cents from the owner of a Springfield GI-45:
Determine what features are likely to be most important to you in the platform, then spend as much money as you can up front, rather than a similar or greater amount down the road in "fixing" or customizing the gun. It's easier to "downgrade" a model with all the bells and whistles - that has had more care put into its initial construction - than it is to take a "starter gun" and attempt to overcome limitations in slide-to-frame fit, etc. To my mind, the frame, slide and barrel (in that order) should be your most important considerations.
Like I said... two cents.