A few auto body questions

The harbor frieght gun will work fine. I have had expensive guns crapout (costing me time and money)on me and cheap ones never. So if your on a tight budget stick with one of there guns just stay within the tip limits of the gun and paint, primers, and sealers that stay in that 1.5 to 1.8 range of the gun and what the paint system requires. I believe thats what size tips come with their gun.

It all depends on what quality finish. I like to go down to bare metal if its bare here and some primer there or paint. I do this because of the mill of paint i want even as possible all over. I also want to know that i wont have failure from old stuff later. You can sware its level with filler and it feels that way and when your color goes on it tells the truth. It can get past blocking too if you use the wrong size sanding blocks.

Hes right dont go from 50 to 180 you will kill yourself. 50 is for heavy fillers trying to shape or strip crap even clean welds. I go to 80 or 100 then go higher from that

Iam sure you have heard this everywhere but prep prep prep is king. I block sand 3 timeas at least. Then the cheap paint they gave me looks nice because of the prep. You can do great things with that cheap gun its a good surface thats key
patience.
Hope that made since