Yes, everything can be upgraded whether you mean jump distance (Frame Shift Drive) or speed/manoeuvrability (Thrusters) or the best upgrade you can get first (Power Distributor).
Keep an eye on power-plant output. If the power used by all upgrades you would like exceeds output, then you might not be able to use them when you need them.
I'd upgrade power-plant also as needed.
But if you are upgrading a sidewinder for a bit more jump range, you should find around a 12-13LY range good enough for most things.
I'd upgrade the ship. I quite liked the adder in the smaller ships range. nice little 'scrapper'.
Hauler is a good upgrade if you plan to run away from trouble. Lots of cargo and a good jump distance as standard.
I'd avoid the eagle unless you have money to upgrade FSD and power plant at same time.