It's also pretty easy to flip the scarifiers once you know the drill. Woods used to have the manual online, don't see it now but I think I have one downloaded.
From that you can use all the estimates here to try to figure an offer that won't be so low as to be insulting, though if it was me, I'd make a very low ball offer and be ready to (pretend to) walk away, and also plead ignorance if he seems insulted by your offer.
My previous tractor was a grand L3240 with loaded R4 tires. I had (still have) a Cammond box blade. I bent the blade with my little tractor but when I bought my blade, I didn't realize I was getting the 'economy' model. It was just so pretty and orange, I just had to have it. If I remember correctly, my blade is just over 600 lbs. I have ALWAYS wanted it heavier. You tractor will have NO PROBLEM lifting the blade and yes, it will stop the tractor if you load it up. Just don't load it up. Learn to work the blade and weight will help you at times. 6 Foot will cover your tracks.
My 2 cents...
PS You will be surprised just how much your tractor will pull
I contentedly pull a 60", 630 pound 'Bush Hog' Rollover Box Blade through Florida sandy-loam, behind my B3300SU. (33-hp/1,800 lbs./industrial tires) So the 900 pounds BB you are considering is in scale to your heavier tractor.
I rarely put down the rippers; weight accomplishes the work using the scrape blade alone most of the time.
Be a little cautious when pushing dirt in reverse. It is possible to deform lift arms and check chains operating in reverse, as three point hitch is engineered primarily to PULL not PUSH.
First picture below is my ROBB pushing loose dirt/ashes in reverse at our community burn pile. Note: Kubota check chains replaced by pin-adjustable, heavy, telescoping stabilizers from Stabilworks for reinforcement to three point hitch components.
Kubota tractor stabilizers Kubota www.stabilworks.com - YouTube
I use an 84" HD box scraper and as stated when loaded it will stop my JD4510 in it's tracks. But, once you learn to control the feed rate into the box by adjusting the cutting angle and elevation you will not have a problem. This takes some seat time but that's what it's all about.
for the HD blade shown... id like to get it for $500 (under that unreasonabl expectation) LIkely offered for $1200-1500 in my neck of the woods. Wouldn't spend more than $1000 if it was in good condition, would try to get it for $800.
At $800 id feel i got a good deal and the seller didnt get ripped off. @ 1000, Id think the seller would be pleased and the buyer shouldnt feel like they overpaid.