I'm not a pro in this subject matter but since no one has replied to your post I'll give you what tips I can.
Removing the old paint job can be a real pain in the @$$ but this will save you the greatest amount of time and cost through a body shop if done properly. What you need to know before you start sanding is how deep you plan on sanding the 'DIY' paint that's on it now. If you go all the way down to the metal, you'll have to cover it with a primer/sealer or you'll get surface rust practially overnight.
If you want to take it down but not too far, I'd start with 220. I think 180 would be a bit too aggressive if you aren't using a primer. I'd also use a d.a. sander to get an even sanding surface. You should be seeing old primer and original paint pop up pretty quickly without using too much pressure. Work your way up to 320, then 400. I wouldn't go past 400 if there is bodywork to be done. Just remember that when you are sanding, you'll see bare metal FAST on edges and seams so be careful around those.
Honestly, this is a lot of work for you to take on if the car is being dropped off at a bodyshop anyway. If they are doing bodywork and primering and such, then find out if they even want you doing this. You could be giving them a trashed canvas to work with if you know what I mean. They may want it as it is so they can do what they do best, and I know that it may cost you more, but it could allow them to do a better job. I hope this helps and good luck with your project. I want to see some pics as you make progress.