I guess I'll be the dissenting opinion here. They're Spyder clones, so just about anything from an older Spyder should fit. O-rings *should* all be tank o-rings, if I remember correctly. But I have Spyders from the 1990's that have the original o-rings. These aren't that old, and are probably fine. Yes, you'll need a 7/8" elbow. Get an APP Atomic Elbow or Pro Team Products Pro Feed.
You're correct that there is no gas-through grip, expansion chamber, or regulator. Just a standard bottomline. If you feel the need to add one, you can pick up a used one for a few dollars. Or you can hold onto that hose. It's stainless steel, you're not going to hurt it or "fray" it. It's better than macroline.
Yes, this is a discontinued marker, but it's exceedingly common and a clone of the most common gun out there. You won't have any trouble finding parts for a long time, and it's so simple that you shouldn't need to replace anything other than o-rings, springs, and maybe a ball detent.
This is leaps and bounds ahead of some poorly-maintained rental and would pay for itself in 2-3 trips to the field. At the very least, it's worth having as a backup or loaner. They don't get much simpler than this so it's a good backup.
If you're really adventurous, you can even turn it into a pump.