That sounds like a pretty good deal and it is very doable, but it really depends on what your building skill set is and how much money you want to throw at it.

If it were me, and much of the glass is already missing I would look into replacing it with double wall polycarbonate. Its cheaper than glass, easier to work with and can be cut to size on site.

As for making it a stand alone and replacing the 4th wall, I would look into what it would cost you to get aluminum U channel in your area. That is how I would reconstruct the 4th side running 2 pieces parallel to the ground 1 at about 18" off the dirt, and another about 5' of the ground (of course depending on how tall the structure it these numbers could change (but since you said it was mounted to a house I would guess its roughly 8' tall on that side.

If you could post some pics it would help alot with giving you more details, as I could then see how the structure it setup.

A couple of thoughts...

First, 5 years ago, my 20' x 23' tempered glass and aluminum freestanding freespan GH was $15K. My was a higher-end GH, so I think your [new] price is closer and his is high.

ggarner has a point on the polycarb. Energy cost-wise and in terms of light diffusion, the PC has an edge. My tempered glass needs some light diffusion to prevent plant burn.

I'd try to negotiate a lower price, honestly. You may put $5-10K in a foundation for this house. It might cost him at least this much to have someone take it down.

I'd pass and look for a small used commercial poly house instead. You can probably pick one up for under $3K and it will include lots of components like heaters and fans.