18 PTO hp is a bit on the weak side for a
chipper with 36 acres to tend too.
I also agree that the small feed can and will make feeding very difficult without trimming.
I would suggest considering a larger used gas or diesel
chipper.
My son & I purchased a Vermeer 1250 (12 inch) powered by an 85hp Perkins diesel for $5k to clean up our 8 acres and I don't regret it.
This is a 6000lb machine and will chip your 4 pine in less than 30 seconds without trimming and 8+ inches with minor trimming of the lower branches.
It is easily moved with my Ford 1720 or a 3/4 ton truck.
You will find it less work, more productive and you can disconnect it and use your tractor to feed the
chipper.
Working alone using the tractor I can cut drag and use the bucket to set the trees on the feed apron and push the tree into the feed rollers.
I'm 56 and have had numerous major health issues including 2 liver transplants since 1989 so making a job easier is absolutely essential for me to remain productive.
Not knowing all the details and your particular wants & needs including the lay of the land I thought I'd share my experience with you.
I like PTO powered attachments also but sometimes a self powered unit is the way to go if all aspects of the job are considered.