I’m no De Rosa expert, but their esteem is obvious. Even though this frame is in my area, I wouldn’t want to tackle this restoration for 300. The reason being that because it is ‘such’ a desirable bike, I personally would want to have the frame restored back to original condition, and ‘that’ is going to be expensive. The TT cable guides appear borderline toast, although I suppose a good frame builder like Paul Sadoff in Santa Cruz or Ed Litton in Richmond could replace them. Also, I don’t think there is much you could do about the chrome that has flaked off the chainstay other than paint over it or have it rechromed. If you were to have the frame professionally refinished, it could easily cost 600 or more.
The other way to go, of course, is to DIY with an OA bath and touch-up paint. I just wouldn’t feel right doing that to this level of bike. If you can talk the seller down to something more reasonable, or if the seller would throw in some of those STI components, it might be worth a go IMHO.
(Disclaimer: I'm still a rank amateur, so take my advice with grains of salt please.)