Hey, Matt...
I concur with jpmorris...the 58 uses the same 8-track headstack as the 38, 48, 58, TSR-8 and ATR60-8...You may even still be able to buy them new from Teac. And unless you are using a machine 24/7 heads last quite a long time especially if its just home or project studio use...lots of opportunities for spares out there for those headstacks.
To be honest, those heads look good. I always look at the lifter posts...you can't typically rotate the lifter posts, or if you can its not at all easy, so unless the lifters have been replaced or have had sleeves put on them (and you can usually see a sleeve...not very common), I usually look to the lifters as a key clue to whether its a low, nominal or high-mileage machine, and I don't get more granular than those categories because the ONLY way to know the life remaining on a set of heads is to send them off to somebody like John French at JRF Magnetics and have them analyzed. Period. That's it. Everything else is, at *best*, a bit of an educated W.A.G. But if the lifters look like they've got low miles on them I look to see if that wear corroborates with the guides...and yes, as jpmorris points out, guides can typically be easily rotated (and this is true of the 58), so its easy to hide ugly wear. So I try to look all around the guides for wear paths. But if the lifters look like they've got low mileage, and the guides show the same (if there are multiple wear paths then kind of add them up in your mind), then its usually relatively safe to assume the heads are the same. The caveat is you can replace the guides and lifters. I bought a new set of guides and lifters for my first 58...never installed them...I still have them...will likely put them on my second 58 when it comes time for a relap...anyway...I'm not meaning to be complicated here but want to make sure I cover the caveats because they are many...heads can look great but if they are relapped and have had new edge slots cut, well...they can *look* brand new but have 50% or less life in them, or less...often less. I sort of did a poor man's optical analysis once on a 2" 16-track "black-face" Ampex headstack...its in one of my threads around here. That's the only head I've ever had that one could even do anything like that because there isn't any material in between the head coils in the stack...where the guard bands are...so you can actually see the head coil tip, and that's the only way to know the actual wear...you have to know how deep the tip depth was when new, and optically measure the existing depth...divide the existing tip depth by the original tip depth and you have your percent remaining. I have a near new 1" 8-track "black-face" Ampex headstack (same construction between the 2" 16-track and 1" 8-track stacks) and used a magnifier, really good light, and a feeler gauge to compare the depth of the new headstack to the depth of the worn assembly and estimate the remaining life, but I still had to assume a large margin of error to the lab equipment John French uses. but it was effective enough to answer what I needed answered...I bought the 2" 16-track headblock assembly with a return guarantee, and after determining that, for certain, the headstacks had considerably less than 50% life remaining, I returned them for my money back...seller said he thought they were "near new"...and they did look nice...but they had been relapped...another obvious giveaway sometimes is there will be a sticker on the heads they've been relapped. JRF has a "relapped" sticker. But its a sticker...they can be removed. But I digress...You can't do any of this poor-man's optical analysis with the Teac headstack anyway.
So from what I can see (and I kind of don't like passing judgment from a couple pics online...you are getting my unprofessional opinion only here and YMMV, buyer beware, etc.), the lifters have light wear, the surface of the guides that is visible appears the same...the profile of the wearing face of the head coils has a very similar profile to the surfaces outside the edge slots, and the edge slots are still deep (again, if a relap is done right on these headstacks the edge slots are re-cut, but that's not always done so sometimes you can identify heads have been relapped because the edge slots are shallow and narrow)... I don't see any keystone or hourglass wear pattern on the face of the headstacks [EDIT: scratch that...see wkrbee's comment below...he caught the trapezoidal wear on the record/sync head...I missed it...but see my additional comment after that below]...in fact its hard to see any wear path from the pics...I don't see any open coils...I also don't see a bunch of nasty crud built up on the mounting block...based on what I see I'd say that machine has relatively low miles, has not likely been relapped, and looks like its been maintained.
I wouldn't worry about track 1 being low output...understand it is best to assume you *will* have repairs and maintenance to do before the machine is 100%...just expect it...then you won't be disappointed when you are faced with the reality. If you aren't comfortable repairing, setting up and maintaining something like a 58, then strongly consider finding a tech within reasonable driving distance from you. But track 1 being low output could be:
1. A speck of something on the head (it takes *very* little foreign matter to considerably diminish the output level)
2. The machine needs calibrated (somebody didn't do it right and one track is vastly different than the others...it happens)
3. A relay is going bad
4. The amp cards need re-seated
If you get it, first thing to do is to clean the tape path, re-seat the amp cards and roll some fresh tape...see what you bought. Next up is a full calibration. Then you *really* know what you have. If the heads are near end-of-life you'll have a difficult to impossible time getting the record and reproduce frequency response dialed in within spec.
But if the price is right...I think it is far from being a train-wreck, that machine.