Sounds like it is past due for a major overhaul, not sure if it really needs new derailleurs, probably due for a chain and chainrings and freewheel, new cables and housings, and a full teardown and cleaning/regreasing.
Rebuild existing vs buying something new is your call, as is buying something used but newer. Buying something new/newer then gives you the option of selling or keeping the current bike and possibly reconfiguring it to a different format such as a internally geared rear hub, or a single speeder, or yard art.
I can't often afford new so I'm usually in the "fix it or buy something used and newer" category. New bikes open you up to all sorts of new concerns though such as cartridge bottom brackets (one daily commuter I used to work with goes through at least one bottom bracket a year, and now is also looking at needing new cranks after maybe 3-4 years due to the crank assembly to bottom bracket interface (design flaw)).
Since you have five bikes it may be easier to just modify one of your other bikes to replace the worn out one (street slicks on a MTB as an example, or adding racks and such).
Hope this helps a little in your thought process.
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