The two frames in question are very different things in my mind. A nice SLX frame in great condition may be worth the 200 euro, but that is on the pretty high end of the scale of what I'd pay for something I can't visually inspect and handle before buying. I doubt I'd pay 100 euro for a rough condition aelle frame.

All markets are to some degree local, so you'd have to see how that compares to what you can find locally. There was a thread on here some time ago where someone in your corner of the world asked over the course of a month or two about bikes they'd found in local sales listings. Some of the bikes were quite reasonably priced for their condition and quality. Considering your employment situation, you may have the time to spend shopping around for a better deal than either frame.

BSA and ISO are interchangeable bottom bracket threadings. See Sheldon Brown's website for a myriad of info about how to identify parts/threading/etc/etc You can usually quickly tell if you have an ISO/BSA bottom bracket because the threads will "oppose" when you put the two threaded parts (facing the same way) next to each other. French and Italian BB's are right hand threaded on both sides, whereas ISO is left hand threaded on the drive side. Chances are your mid 80's shimano BB is ISO, but you should be able to tell quickly (either by it's markings or looking at the threads)