Here's my take on it, if it's all exactly what you want (other than maybe the spring rate), then you should make a play for it.
It's a lot of money and a lot of work that you do NOT want to do twice, just to save some money on the front end.
K members on facebook marketplace are a dime a dozen.
Don't be banking on selling parts from the kit, because you will take a hit, just like he's going to if he sells it to you.
Personally i'd want a 1/3 off the new price to be worth my trouble. And you really need to know the spring rates and if that's what you want, because it could cost you another 100 bucks to change them, shrinking the deal.
The higher number is stiffer, the lower the softer.
Easiest way to look at is is 150lb is more of a drag race number and 300lb is more of a road coarse number (the numbers get more extreme, just not really for street cars).
I run 275lb.
Edit: I wrote the paragraph above earlier and didn't post it. Now that he has offered $1100 that would be a firm no for me.
Let him find some other guy (which he won't) that's willing to let him off the hook with only a 20% hit.
4 hour drive to save $300? Certainly not enough motivation for me.
When he comes back at you agreeing to the $1000, tell him you spent 100 bucks and now you only have 900.
The $1000 was more than fair (as i said i think too generous), he should have taken it.