The Trilever (marketed as Tri-Leva) version of the ML7 isn't very common, and is a very effective addition., hence prices are a bit silly.
ML7s (and all their derivatives) have become very fashionable machines so tend to be quite expensive for what they are.
That said, they are a good (groundbreaking at the time) machine, and practically all spares and accessories are readily available (again a bit on the pricy side)
The main issue to look for is bed wear, which generally occurs at the headstock end, and of course if you adjust for wear there it'll be too tight, and jam up further along the bed.
As for prices a lot on Ebay really try it on, but looking at a Tri Leva unit sold recently by a machine tool retailer, with a Myford cabinet stand 3 phase power and a lot of accessories, that was £2200, but an offer was accepted which I imagine was around the 2k mark.
I've just paid £400 for a bog standard ML7 that is also going to be a minor restoration.
So it's difficult to really say what's a realistic price, simply because it's a fashionable item with a relatively rare and desirable feature.
Just to add, the serial no is on the back side of the bed at the tailstock end and should start with a K.
