You would probably have to clean and grease beater bar bearings (dc04) and if clutched probably rebuild the clutch mechanism - i would anyway. Belt and post filter is a minimum requirement too
Dc14 clutched are good imo, sold many over the years, solid reliable machine.
Dc25 i like too, but at their age can be troublesome with beater bar motor, change over valve assy, yoke cable and beater bar vibrating due to wear.
If the refurb option, ask how has been refurbished, (warranty) ? for example - i and most others here on a refurb would totally strip a machine, replace motor / brushes, gaskets (dc25 cyclone defo), beater bar bearings cleaned and regreased, belts replaced, clutches rebuilt, everything cleaned etc.
Beware of the refurb wipe over brigade.
As for other models / brands sebo are good, new have a 5 year ticket i believe.
Our main vac is a dc24, small and performs well but again anything mechanical will fail eventually.
Our secondary vac is a cordless v6, on its second battery but been reliable, with basic preventative maintenance.
Newer dyson models dc41 upwards i wouldn't go near but thats my personal opinion, other members may well advise differently.
Dc01, 04, 07 and 14's are built like tanks and last years.
Do not give the DC18 a second look.