So the old fabers aren't something you have to worry about, I can honestly say I've never actually seen a medium pressure Faber tank in person. Obviously they exist somewhere, but I have never seen one nor have I ever heard about them other than "they exist", so don't know anyone who has or had them at any point in time. Should also mention the PST e7 and e8 series tanks do exist, I've actually seen those, but they are rather hard to find because of the limited run time they had, you are far more likely to fine the true HP100's.
The "HP" tanks are difficult because HP100 refers to one very specific tank, and that is the PST 100 with a service pressure of 3500 psi, all the others are E7-100's which are very different tanks. Even when PST stopped making the HP100's and went to E7-100's they changed
PST E7-100 | 3442 PSI | 100 | 7.25 | 24.12 | 33 | -1 | -8.5 |
PST 100 | 3500 | 102 | 7.25 | 23.94 | 34.1 | -1.3 | -8.8 |
Worthingt | 3442 PSI | 99.5 | 7.25 | 22.7 | 33.1 | -2.5 | -10 |
Faber 100 | 3442 PSI | 100 | 7.25 | 25.3 | 34 | -0.6 | -8.4 |
The XS ones are a different tank than the PST's, and the fabers are even more different, instead of shorter and more negative, they're quite a bit taller and less negative.
So, what does this all mean. Tank sizes don't mean jack in this country, even with LP85's, there is a huge difference between the old ones and the new ones in buoyancy from Faber *the new ones float, seriously, that's why DRiS doesn't sell the LP's, only the HP's. I would hold out and try to find the old PST tanks used, they are available and are wonderful little sidemount tanks. The LP85's are nice, but don't give you as much of a lead advantage in the cold which is why your instructor recommended the Worthingtons because they are quite heavy. I greatly prefer PST to Worthington, but that's just me, I also don't dive often in locations where I need any lead with my PST's.
Scuba Tanks - Steel 100's High Pressure
PST HP 100's, you can find a righty valve for them, $200 for the pair, not bad
Scuba gear
LP85's, bit expensive, but you can probably talk him down to $200 each since that's the going rate
Scuba gear and equipment
those look like LP95's which are also good for sidemount if you like shorter tanks, they're 8" diameters, but some people don't mind that. Lots of guys in the Marianna area dive 8" bottles.
2ea steel scuba tanks 108 cu ft
108's, bit longer and wider than the 100's but they balance really well for sidemount, this is one of the more common tanks sizes in the Marianna area for sidemount.
I dive PST HP120's or LP72's for most of my diving, I don't like 8" bottles so I don't own any. If I were you I'd see if I could get those PST100's I linked for $350, then you can find a set of lefty/righty valves for them and you'll be good to go.