That camper looks OK. Other tricky parts can be the lift system and bed slideout hardware. Look for hidden signs of water damage. Especially in the roof corners and floor corners.
Canvas Replacements is a pretty nice outfit (even though they aren't the best canvas folks). Other sites source parts from them. They are also good folks and helpful if you give them a call. I'm sure if you called and said you were looking at an 85 Coleman and you were worried about part availability, they would be happy to talk to you. Like I said, they've been great and they have a 100% satisfaction guarantee.
But I have to warn you, lift system and bed slideout parts very specific and expensive. If those 2 things work in this camper is could save you about a grand in restoration.
If you're thinking of rebuilding, then you should check out
Pop Up Portal. Nobody knows more about lift systems, slideouts, leveling, etc than them. However, most folks there have never camped on anything more than a cement pad. That's why I spend most of my time here.
Coleman was smart and integrated the lift arms into the main frame (unlike Starcraft where the lift arms all punched through the floor).
Also, you should be able to get him to come down on the price. I'd say $300 is a can't miss and $400 is reasonable, I might still do $500 if everything is in working order. You're basically buying the frame and hardware.
Also, a frame up rebuild takes a LONG time and adds up. If a good E1 falls into your lap you should consider it. I haven't seen any in your budget, but it's worth a look. My original intent was to redo the inside and maybe bigger wheels, but when I realized it was rotted I knew that what I wanted, nobody sold. So I'm slowly building it.
