Felipe, you didn't say (at least in this post - I have NINE pages of back-log to catch up on!) if you got the car or not. Two things that haven't been mentioned yet...
1. The steering box. On my '72 (that I had way back when) I got it with a "pulling to the right" problem, but the price was so low I would have been stupid to NOT get the car. I lived with it for awhile then took it in to a great Fiat mechanic that used to work not far (VERY relative term) from where I lived at the time for other work (valves) and asked about what the pulling issue might be. He took it out for a drive after the valve work, told me what - in his opinion - it was, and for a few hundred (less than $300) it was back on the road and better than ever. And that included the valve work!
2. The cross-member mounts. On both my old '72 and my current '76 the cross-member under the motor is (was) not in the best of shape. It turned into a catastrophic failure on my '72, and now that I know what I'm looking at, will get fixed on my '76 in short order - again a relative term. It's not so much the cross-member itself but where it mounts to the frame. That area has a tendancy to rust out and then tear the mounting points from out of the frame. A good welder can fix it right up for you as long as the frame itself isn't compromised too badly. But even that can be fixed...
Just a few things to think about, and if the price is right I would jump on a nice Spider in a heartbeat! Makes a great addition to the stable alongside an X...