When you say "they are bad" - are they rusty, or is the paint just crap?
If they are rusty - it is structural and you need to get the mig out. Could be expensive.
If they just look carp, then make sure they don't go rusty, and then wallop some standard veloce skirts on. Or Zender, but you can probably get standard items in the right colour for a few quid.
What I am doing on the project car is getting the MIG out, fixing the metal, rust proofing it, then walloping a set of Veloce skirts off the parts car onto it. It means I don't need to spray the sills, and any imperfections in the welding will be hidden.