Most materials fatigue with use and age. Since you didn't mention the year it's a tough call. My beloved Candy Girl, hate to say it, is at that point in her chromoly career that she's a bit, squishy. I can stand up in the saddle and I swear her bottom bracket hits the ground and rebounds. Poor old girl.
So that is a consideration, but the other thing is building up this new frame. Building is expensive. I work in the industry and seriously spent an entire night CRYING because the bike I wanted was going to cost anywhere from $1000-1500 more than buying similarly equipped, but not as nice frame wise bikes (and damnit I wanted that bike, and I wanted to name him Kermit). All the deals and discounts in the world could not make building practical.
Even if you plan to strip and old bike and rebuild the new frame with those components, if you can't do it yourself figure on at least $100-$150 each direction, tear down and rebuild.
Then that leaves you used, older, Al frame. Can you get enough out of that to demo it in the name of the new frame?
Now then, all these reasons not to do it aside, I think it depends on what you want from it. If you're going to ride the daylights out of it, I don't think this is a great idea. If you just want lugged hotness in your stable for now and again well maybe this would do, but you're also tearing apart a workhorse to make a Sunday driver.
My other concern is why did he tear this bike back apart? Selling components can be fairly profitable sure, but what a pain to tear it apart and sell it piecemeal.
I'm going to revert back to my experience in the horse world here. If someone relatively inexperienced told me they were going to try and buy a foal and train it themselves I would advise against it. Newbies belong on solid 10 year olds that still have spunk, but stop being stupid a few years back.
The risk here is too great in my book. You're buying something you can't ride and worse, at least as I understand, tearing down a bike you might decide to put back together.
I *heart* steel to no end and I think if you dig it go for it, but this isn't the best route for you at this time in your cycling career. I know you had mentioned previously you wanted steel. I'd say check out Masi and believe it or not Raleigh this year. Both are turning out some ADORABLE steel bikes of late. I'm torn between keeping Candy Girl and getting a Masi Soulville. Mmmmm those are hot.
"True, but if you throw your panties into the middle of the peloton, someone's likely to get hurt."