If it does not run, it's worth $6-900; that's what the parts are worth.
Check out John Smith's page: http://www.oldfordtractors.com/qa.htm#q16
I've bought 4 N's using John Smith's checklist; it saved me from buying three or four others! The only thing I have to add to John's excellent suggestions are:
1. Take a jack; jack up the front end & check it out just like you would a car for slop in the bearings, tie rods, etc.
2. Take a compression gauge. You can pull all 4 plugs & do it correctly, or roll the dice & do one cylinder.
3. Take a meter in case the ammeter on the tractor is busted; you can see if the charging system is working.
4. Get back home & access an on-line parts catalog or take a paper copy w/ you. Figure out what it is going to cost you to get it running (or looking) the way you want it. If you think it's bad.....it is! Price it accordingly & consider those costs in your offer. For example, if you see in-line gas filters on the tractor, figure $150 in costs for a new gas tank. Same thing w/ a 12v conversion; that's a usual band-aid for a low compression worn-out engine. So, do a compression check. Or walk away. W/ the engine hot & at idle, walk away if you don't see at least 5 lbs of oil pressure. That is unless you don't mind rebuilding an engine in a couple of years.
As to a deal breaker.....you're going to need to do more research on prices in your area. A running 8N w/ decent tires, good hydraulics, no smoke or funny noises will sell for under $2k in my part of VA. So, if it was $2k and needed rear tires.....that's a deal breaker. But, $1,200 & needed rears.....well, that's a different story.
Do not limit your research to N's. You will get a LOT more tractor for only a little more money w/ a 100 series. I have 4 8N's & a 740. The 740 has live hydraulics, PS & more hp. And I paid the same $$$ for it as I have for 8N's.
But.....100 series tractors are rare in my part of VA & N's are very common. So, it depends on what is available in your area.
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