.023 would be best, .030 works if you have everything super clean and some experiance, if not youll warp it pretty easily tho.
'bondo' (bodyfiller) isnt that hard to use. for each golfball sized lump of filler you lay a stripe of hardener across and mix it up until its all 1 uniform color (pink if you used red hardner or light blue if you used blue hardener.) get some spreaders to apply it to the areas to want, and dont get excited and apply too much or you will be sanding forever. im not sure if its correct or not, but filler should only be applied to bare metal or a sanded epoxy primer.
to sand it i start out with 36 or 40 grit on a longboard at 45 degree angles, when you are sanding hold the board at an angle so it doesnt dig in and leave grooves.