My Moto knowledge gets a little fuzzy into the 80s but I don't think any of the Mirage variants used high quality framesets though I could be wrong. If the decal has numbers like 1040 or 2040 or similar it's a hi-tensile frame and pretty mundane. Vitus 888 would be good. Ask the seller about the numbers on the decal.

In any case sounds like the bike has upgraded drivetrain and wheels as I doubt any of those were OEM.

Worth it? If it's a Vitus frame yeah, but you're at the upper end of the price scale to be sure. Some will tell you they found better for 10$ at a garage sale and while I don't doubt them it's not as easy as driving down the street looking for a clean, quality used bike sitting in a front yard.

It's better than the current market will give you for the same money but I'd definitely negotiate. Sounds like the guy is flipping it himself or he'd know more about it.