The only hardware up grades you can make are adding more RAM and HDDs. Since you have indicated that that you will be "heavy-processing tasks", an upgrade to 8GB of RAM would be very useful.
The CPU will be consuming a lot of power. The RAM will in effect reduce battery consumption since it will foster shorter CPU processing time. The battery should be let to discharge about once a month to about 30% + or -. When ever convenient use the power adapter. You cannot harm the battery with heavy CPU usage, unless you drain it to exhaustion. Avoid that. The battery cannot be overcharged.
Large external drives are good. In time you will find that there no such thing as having to much storage space. I suggest a separate drive dedicated solely for Time machine. External drives have no meaningful impact on data processing by the MBP. They are simply storage devices.