Re: Painting bus. Your fiberglass hightop may require some special consideration but not likely. The choice of black is a bad one. If you must two-tone, select light colors. Keep the top white and you won't need as much of a cooler noted in your ather post. I won't weld on the body as you cannot reach the backside of most places to refinish what the heat has burned off - maybe in your area it is dry enough that rust is not a problem? By the sound of it, there may be some significant damage. Since you are trying for an okay look (looks great from across the street!) consider bonding patches with adhesives designed for the purpose.
One of the problems you face with the adhesive approach is initial cost for the adhesive and for the applicator gun. I used a flanging tool I made out of vice grips... Cut out the area being replaced and flange around the perimeter of the hole - an offset to allow the new panel to lay even or slightly below the regular surface. I used a good grade of epoxy from the local hardware store (but epoxy does shrink and does not remain flexible like the polyurethane panel adhesives.) I used flat-head pop rivets along with the epoxy. It's about 10 years later now and it still looks excellent. The filler and adhesive did shrink after a couple months of curing, so a trained eye can see the seam. The rust preventative on the inside of the panels is still intact.
Plan 2 is take some thinner sheet metal and simply pop rivet over the bad area, fill deep enough to blend and hide the pop rivets.
When using body filler, it is important that it has something to really attach itself to. Drill holes, grind massive grooves, do what you can to give it something to grab onto. If REALLY bad, use sheetmetal screws or pop rivets to attach some 1/4" mesh screen (hardware cloth) and fill it with the body filler. This is a good way to span deep fills, too, but make sure it can't work (move). Another way is to use expanding foam to fill really rusted out and deep hole areas (possibly using hardware cloth to suppoert it) then put screen or hardware cloth over the foam and fill with body filler (this method offers support so the patch doesn't "oilcan")
Body fillers drink water. You MUST seal the body filler. Primer is not a sealer - it will allow moisture to saturate the body filler unless sealed.
Maaco is an excellent plan for a low budget.
If you shoot it yourself, buses are easy. Do one panel at a time or alternate panels. Paint seam-to-seam. If you do multi color, use the rain gutter for your split as nobody will see the seam.
Paint outside after the trees and shrubs have ended their yearly sex acts and after the mosquito ponds have dried up. Paint early in the day. No - earlier than that! Spray a light mist of water wetting the ground all around the area where you're painting to keep dust down.
Maaco may use cheap paint, but if you aren't good at this (sounds like it's your first project) chances are your prep is gonna be the killer down the road a few years anyway and a cheap paint job that protects the body from rusting and looks good for 8 - 10 years is better than spending much more on paint and applying it yourself without the proper knowledge.... My Maaco job looked good for over 10 years. I think it was one of the $99 specials! I removed all the trim, lights, etc. It was a '76 Corrolla. Buses undoubtedly take more paint!
Consider - especially if the bus is really rough - one of the many DIY pickup bed liners, especially around the lower part of the body.
All offered herein are my opinions based on cheap, get-by-for-now experience! -BaH