The 28 I currently have (this is my third), is a nice, basically NIB 4" I got for $450 about 6-8 years ago or so. It is amazing how fast the prices have jumped up.
They are great guns, and seeing what the other older S&W revolvers are going for now, $950 seems to be about in line with other things that are similar these days. Decent older S&W revolvers of all models (excluding the well worn police trade ins) seem to be running $800-$1200 around here when you see them. And they usually dont sit long.
If you like it, and you dont mind the porting, it doesn't sound like a bad deal. You could always offer less too and see what they say. Maybe use the porting as a negative.
I had a 4" 29 Mag Na Ported back in the 80's and pretty quickly regretted it. Never saw a reduction in muzzle rise or recoil, but that big flaming "V" was impressive and distracting in lower light. The front sight was always fouled, but you dont notice it near as much as you do on a SS gun. I didnt lose any money on it when I sold it, but porting was a thing back then too.
About 5 years ago, I picked up a used 3" S&W 696 that was a Lew Horton run, and it was Mag Na Ported by them as part of their deal. Love the gun, hate the porting. The biggest issue is the fouling of the front sight, and actually, the whole front of the gun, when using lead bullets. You also dont want to fire it from a retention position either.
ETA: This will give you an idea as to the fouling with lead bullets after a few cylinders full. Theres a red insert in that sight too, not that you'd know it...
Same using plated or jacketed.