I'd want to see the other side and see if it has the old billboard roll mark or new. I would also carefully function test the safeties. That pistol has some unusual modifications.
Looks like aftermarket sights, hammer, and ambi safety and grip safety but factory plastic trigger.
The magwell looks like it may not be fitted real well but I can't say for sure from the pic. Not a big deal I stuck one on a kimber I am going to use for IPSC and it doesn't fit too good either, but it is something to talk about while talking price.
Maybe the owner was a lefty and wanted the bare minimum done to shoot the gun in competition or something?
I'd be thinking around $500 if it seemed to function well and you like the modifications. I think a new in box stainless new roll mark 1991 can be had for around $600.
Those who know more than me should chime in...