Fair enough - In that case if it were me, I'd only upgrade the skimmer.
, I wouldn't go with the fluidised sand bed, UV
filter or external filter they are overkill - someone else may back me up here.
be a good thing - EG: heater, it's better to use a heater that is rated for a much larger tank as it will be more efficient in heating the tank.
At the end of the day, if your budget permits and you see a valid reason for replacing the old - why not?
In regards to the external filter, you won't need it if you have enough LR
and Power Heads to keep the water turing over.
Before you emptied your tank what were you keeping?
Was there something in there to make you think you might need a UV
filter or a sand liquidizer?