I'll detail this journey with the most common answers .
Price, that's a tough one. As with older cars, 40 years is 40 years. You don't know what you have until you know what you've got.
There really aren't a lot of ways to completely disqualify a purchase. I'll cite some of the ones I ran into in my two year search.
Beautiful re-paint, except for one thing. They had painted over the vertical ways. Not being able to inspect the condition of the dovetail I passed on a $3K paint job.
Peckered to hell table. On top of the pecker tracks and marks in the table someone had actually been able to pull 3/8" hold down hardware out of the T- Slot with a sizeable chunk of the table. "It's an easy fix for any competent machinist" says the seller. Any competent machinist would not have abused the machine in the first place.
Zed screw, nut, bevel gears. Since the weight of the table sits on this screw it's difficult to access if the resistance is weight or crud. The weak link becomes the gears followed by the nut and screw. We are fortunate that the community has or at least had documented these parts in the past and since this is a much loved niche machine repairs are difficult but not impossible. Once acquired this is one of the first maintenance items.
Hold down hardware. 5/8" is common, 1/2" less common, 3/8" hardware is camouflaged cheese. You'll be making, sourcing, or modifying your own T-nuts. Other than a very few online retailers, all the hardware is Chinesium. Some may offer that creating a purpose point of failure at the T-nut is a safety feature. I disagree. Stay within sensible limits of the machine and all is well. If you really think that 3" diameter .080" surfacing cuts in steel are necessary you need a bigger mill.
You probably won't get the chance at most face to face inspections to tram the spindle and see if the ways are worn in the middle of travel. You can however get a notion of this by running all three axis to their limits. If you do get to seriously inspect the machine the price will usually be a bit higher because the seller already knows it's a good one. Don't be surprised if in any axis there is .001" - .0015" or more table tilt in any direction. This is normal running clearance. More on this below.
Expect to do a full spindle service as well as Zed screw service. In order to easily access the Zed screw you will be removing the table anyway. This gives you access to all the screws and nuts. One common modification that has been done to these mills is the addition of a gib lock. This is simply and additional gib screw to lock an axis. Generally the Zed doesn't need a lock. So if you find an extra threaded hole and screw impinging in the gibs this is actually common practice. Lock all but the working axis. This not only reduces the locked axis clearance to near nil but improves the accuracy of the working axis. Not all of the Benchmaster mills had a gib lock . So if you see this extra screw the machine may or may not have been loved (other than purposely not wearing out a gib screw/threads) but the parts that came off it were.
Some helpful links for spindle bearing service.
https://tinyurl.com/2cxhavrehttps://tinyurl.com/55d26knf
A word on shipping. I've used Fastenal Blue-Lane for years. Even though the Benchmaster was made in kalafornia they seem to be found more often in the rust belt. I was able to get a palleted box from U-line for $173 and shipped from San Diego to Seattle for $181. I was extremely fortunate to have family in San Diego and an unusual but outstanding member of Practical Machinist as a seller. Not three minutes into a phone conversation the price no longer mattered and this was the mill I wanted. $1,600 with Vertical and horizontal attachments, two motors, limited but useful tooling, and a conversation that has continued well past the sale. I do hope you are as fortunate.
I as of yet have no set opinion on this modification. It has been reported enough times to mention here. Boring out the MT2 taper to MT3. The reason is that standard MT2 collets are an advantage over a MT2 ER collet holder simply because of working height. Remember this is a pure knee mill without quill. Not only is anything hanging below the lower bearing unsupported and prone to deflection but full inches of Zed are lost. I'm leaning toward not doing this. I don't see me spinning the additional diameters available to MT3 collets. Again, it's a small mill, and there just doesn't seem to be the meat in the spindle to open it up the MT3. YMMV.
Very quickly you will find that although there is plenty of tooling available for the 7/8" and 1" horizontal arbors the one issue is involute gear cutter sets. You will spend a healthy premium for these in 1" and 7/8".
You will then find that these gear cutting sets are very common in 22mm. The 22mm arbors you will find online do not have the off end detail that fits into the overarm support bushing. Other than this they seem to be universally well made. If you or anyone else has an issue with MT2 driven arbors in any diameter Let me know. There is this guy in Michigan...
Sorry for the wall of text and seemingly useless information but it's what I've got.
