With a 6k budget....I'd be looking to the 1980's. Ford 10-series like a 1710.....something like a deere 750/850/950/1050, Massey 230.
You can always find a little older, little newer, or bigger/smaller in your budget. But $6k opens up more doors. A typical "budget" for someone shopping for a N-series is usually like $1500-$2000. Which really limits you to just the N-series machines
For the OP:
I have owned an 1951 8N for exactly 50 years (I am 80).
I still have that 8N, but have not used it in 10+ years.
Don't quite know why I am keeping it, except for the fact that is a cute old machine, and probably only worth $2500-$3,000.
Bought it when I was a poor B-727 first officer back in 1971.
It was what I could afford,( $800) and I needed to maintain my Minnesota driveway.
The advice given in posts above is good advice!
Since you are an A&P, you should be able to rebuild just about anything.
You might see if you can find a late 80's+ Ford that needs an engine rebuild?
In 1974, I paid for the materials, and had my 8N engine rebuilt by a high school shop class.
Your tractor should have a loader (do not buy one without, and then TRY to find a loader).
I would steer clear of the Ford 1700, or 1710 tractors, as parts are becoming hard to find.
I have a 1989 Ford 1920 with loader, and parts seem to still be reasonably available, but who knows for how long.
The Ford 1920 is a very nice older 32HP tractor, that was built (in Japan) over about 12 years.
I use my (restored) 1920 with FEL frequently, though I do have a much larger Kubota L48TLB also.
I think the secret of available support for older tractors is how many of any particular model were originally built.
The more that were built, the more available the support seems to be.
Case in point: You could assemble a new Model A Ford car today, from new aftermarket parts that are readily available!
Try to take your time, and look for the right deal.
Whatever you do......... try to buy a tractor with live PTO, power steering, and a loader!
You can fix whatever is broken!