I will tell you why these ordinary metal sheets get rusted. It is because they are not treated to be rust proof. Believe me if rust proofing is done prior to application the thin metal parts can last really long. Alternatively you can buy some brand new metal sheet with your denter from iron market and where the small patchwork is required ask the denter to apply the primer to the side that is going on the underside than after application primer first than surface making with puttin than another primer than wet sanding and paint.
For parts that are going to go underside of car like wheel arches and under body, try to get some cosmoline and from gun shop and apply that on the metal sheet instead of tarcoal, After cosmoline it can be painted and will give the look like original daanay dar of honda original.
For color I told you earlier that keep materials in your hand i.e pay the denter painter only for labour and buy paint thinner yourself. Go to any computerised mixing shop it can make the color you desire. Choose the best brand of colour and thinner and also go through the mixing ratios, digest all that information and vomit on your painter. Remember that for the paint to last long the mixing ratio has to be correct, plus the correct number of coats, with correct spraying pressure and distance. I think 4 coats will do good but you have to give a gap of 6-8 hours between the coats, with slight dry sanding by a micro sand paper before every coat. It can be done like this surface preparation at night and than after dry sanding with micro sand paper, apply the first coat and than go to sleep. Next morning start early with dry sanding and apply the second coat, than leave the car in the evening again dry sand and apply the third coat, than 4th coat again in the morning. Now that is done with base coat now you have to go for top i.e laquer
For top coat do sanding before the first coat and again apply 3-4 coats with 1-2 hours gap(I am not sure which sanding will work here but I think it will be wet sanding)
After all the coats of base and to coat done, wet sand all the surfaces than apply a good quality compound.
For this part @margallar is the man
BTW 3000 for the panels is way too much. There are 4 in doors and one big part in the center console and ash tray in the rear. Ask him about his labour. My mechanic takes 500rs for fuel pump change and he opens the entire center console for that. opening the door panels is no big deal. The material will cost you around 500 rs. So I think 1500 rs is good for this work.
If you painter is demanding 3000 rs for this job that is hardly a 2-3 hours job for them, than he sure is an expensive mechanic.
BTW please post the pics of every part to be treated, you will be able to get more advises that way